FABRICATION AND TESTING OF A HIGH-TEMPERATURE PRINTED CIRCUIT HEAT EXCHANGER

FABRICATION AND TESTING OF A HIGH-TEMPERATURE PRINTED CIRCUIT HEAT EXCHANGER Mingui Cen, Xiaodong Sun *, Ricard N. Cristensen Te Oio State University 201 W 19 t Ave, Columbus, OH 43210 cen.3370@osu.edu, sun.200@osu.edu Isaac Skavdal, Vivek Utgikar University of Idao Idao Falls, ID 83844 Piyus Sabarwall Idao National Laboratory Idao Falls, ID 83404 ABSTRACT One of te very-ig-temperature gas-cooled reactors (VHTRs) missions is to produce electricity and provide process eat for applications wit ig efficiency and ig safety. Te electricity generation or process eat applications of VHTRs greatly rely on an effective intermediate eat excanger (IHX) tat transfers eat from te primary fluid (i.e., elium) to te secondary fluid, wic can be eiter elium, molten salt, water/steam, or supercritical carbon dioxide. Te IHX performance is directly related to te efficiency and safety of te overall nuclear system. A printed circuit eat excanger (PCHE) is one of te leading IHX candidates due to its ig compactness and effectiveness, as well as its robustness. In te current study, a reduced-scale PCHE is fabricated using Alloy 617 plates for te eat excanger core and Alloy 800H pipes for te eaders. In tis paper, PCHE fabrication processes, i.e., potocemical etcing, diffusion bonding and brazing, are described. Tis PCHE as eigt ot and eigt cold plates wit 11 semicircular wavy (zigzag) cannels in eac plate wit te following cannel dimensions: 1.2 mm ydraulic diameter, 24.6 mm pitc in te flow (stream-wise) direction, 2.5 mm pitc in te span-wise direction, and 15º wavy pitc angle. Te termal-ydraulic performance of te PCHE is investigated experimentally in te ig-temperature elium test facility (HTHF) at Te Oio State University. Te PCHE inlet temperatures and pressures are varied up to 350 ºC/2 MPa for te cold side and 700 ºC/2 MPa for te ot side, respectively, wile te maximum mass flow rates of elium on bot sides of te PCHE reac 30 kg/. Te corresponding maximum cannel Reynolds numbers for bot te ot and cold sides are about 3,000, covering te laminar flow and laminar-to-turbulent transitional flow regimes. Comparisons between te obtained experimental data and available empirical correlations in te literature sow tat bot te ot-side and cold-side friction caracteristics of te PCHE wit te wavy cannels follow te trend establised in te empirical model well, wile relatively large deviations are presented in te low Reynolds number region. Heat transfer caracteristics obtained from te models tat are available in te literature present some discrepancies from te current experimental data. Large deviations in eat transfer appear in te low Reynolds number region as well. A new eat transfer correlation based on te experimental data as been subsequently proposed for te current wavy-cannel PCHE. KEYWORDS PCHE, termal-ydraulic performance, compact eat excangers, VHTR, diffusion bonding 7622 7621

1. INTRODUCTION Advanced nuclear reactors suc as te very-ig-temperature gas-cooled reactors (VHTRs) from te Generation IV Program are endowed wit electricity production and industrial process eat capabilities. Te VHTRs are designed wit te capability of delivering ig-pressure, ig-temperature elium to a power conversion unit (PCU) for electricity production and an industrial plant for process eat application. Te elium temperature at te reactor core outlet is designed to be 750~800ºC during te first development pase and is expected to be increased in te later development. Wit suc ig temperatures, VHTRs offer a wide range of applications. Te electric power production may use Rankine cycle wit a ig-pressure steam generator, or a direct Brayton cycle gas turbine using te primary elium coolant as te working fluid, or an indirect-cycle gas turbine using a secondary fluid, suc as elium or supercritical carbon dioxide [1]. Te process eat applications may include ydrogen production, petroleum refining, bio-fuels production, and production of cemical feed stocks for use in te fertilizer and cemical industries [2]. Te electricity production and process eat applications of VHTRs are critically dependent upon an effective intermediate eat excanger (IHX), wic is a key component in transferring te termal energy from te primary coolant to te secondary coolant. Te IHX serves to isolate te reactor system from electricity generation and process eat application plants, and terefore must be robust enoug to maintain te system integrity during normal and offnormal conditions. Since elium typically as a low eat transfer capability due to its low volumetric termal capacity and low termal conductivity, a compact eat excanger wit a ig surface area to volume ratio (generally, iger tan 700 m 2 /m 3 [3]) is preferable to be employed as an IHX in te VHTR. Te printed circuit eat excanger (PCHE) stands out from several eat excanger candidates due to its ig effectiveness, ig robustness, ig compactness, and its ability to witstand ig pressures [4]. PCHEs are plate-type compact eat excangers in wic flow cannels (typically, cannels wit a small ydraulic diameter) are etced into flat metal plates using a potocemical macining process. Tere are several types of PCHE wit respect to te cannel geometry, suc as straigt cannel, wavy (zigzag) cannel, S-sape fined cannel, and airfoil fined cannel. Te etced metal plates are ten grinded and lapped to remove scratces on te plates and make te etced plates flat and parallel. Finally, te plates are stacked togeter wit a prescribed arrangement configuration and diffusion bonded to create a igintegrity solid block before flow distribution eaders are attaced to te eat excanger block. Over te last decade, extensive studies related to PCHEs ave been conducted in te U.S., Japan, and Sout Korea. Dostal [5] adopted a PCHE in a supercritical CO 2 Brayton power cycle to perform a system design evaluation in te next generation nuclear plant (NGNP). Gezelius [6] investigated te intrinsic caracteristics of a straigt-cannel PCHE tat coupled a elium-cooled fast reactor to a supercritical CO 2 Brayton power cycle. Cen et al. [7] performed a transient analysis of a fluoride salt-cooled igtemperature reactor (FHR) coupled to a elium Brayton power cycle using a PCHE-type secondary eat excanger (SHX). Nikitin et al. [8] investigated eat transfer and pressure drop caracteristics of a 3-kW PCHE wit a compactness of 1050 m 2 /m 3 in a supercritical CO 2 loop. Ngo et al. [9] investigated experimentally te termal-ydraulic caracteristics of PCHEs wit wavy (zigzag) cannels and S-sape fined cannels. Bot PCHEs ad iger eat transfer performance tan te conventional sell-and-tube eat excangers. Te PCHE wit wavy cannels gave a 24-34% iger Nusselt number tan te PCHE wit S-sape fined cannels. However, te pressure drop wit te wavy cannels was 4-5 times larger tan tat in te S-sape fined cannels for te same Reynolds numbers. Kim et al. [10] designed a PCHE for a 600-MW t VHTR and te PCHE was optimized based on analyses of te capital cost and operational cost. Several factors, suc as geometrical parameters, reactor termal duty, lifetime, and working fluids, were analyzed and included in te optimum sizing model. Kim [11] conducted compreensive numerical and experimental studies on a wavy-cannel PCHE based on elium-elium, elium-water, and mixture (elium and CO 2 )-water working fluid combinations. Correlations for calculating te Fanning friction factor and Nusselt number were proposed for wavy-cannel PCHEs based on different cannel angles, 7623 7622

pitces, and ydraulic diameters. Yoon et al. [12] concluded tat te wavy-cannel PCHEs ad te best performance over oter cannel geometries and configurations due to te wavy cannel s ig eat transfer capability, but low pressure drop caracteristics in te laminar flow operating region. Mylavarapu [2] designed and constructed a ig-temperature elium test facility (HTHF) tat can facilitate PCHE testing using elium as te working fluid at temperatures and pressures up to 800ºC and 3 MPa wit mass flow rates ranging from 15 to 49 kg/. Meanwile, two similar straigt-cannel PCHEs were fabricated using Alloy 617 plates wit Alloy 800H eaders and were installed in te HTHF in a counter flow configuration for testing termal-ydraulic performance. Te autor proposed correlations for determining te ydrodynamic entrance lengt in a semicircular duct and te friction factor in te ydrodynamic entrance region for laminar flow in te semicircular duct based on te apparent Fanning friction factor. Most of prior studies focused on te termal-ydraulic performance of commercial PCHEs under lowtemperature conditions. Limited researc as been conducted on te performance of wavy-cannel PCHEs under ig-temperature elium conditions. Most of te researc used acquired PCHEs from a commercial vendor wit agreement of not to perform a destructive testing on te PCHEs. In te process of experimental data reduction, large uncertainties may be encountered in correlations due to te detailed geometrical parameters not being made available to te customers. Te aim of te current researc is terefore to address tese issues. In tis paper, a detailed fabrication procedure of a counter flow PCHE wit wavy cannels tat can be used in ig-temperature, ig-pressure conditions is presented. Termalydraulic performance testing of te wavy-cannel PCHE is conducted in te HTHF at temperatures and pressures up to 700 ºC and 2 MPa, respectively. Te experimental data are compared wit available correlations for te wavy-cannel PCHEs. 2. PCHE THERMAL-HYDRAULIC DESIGN Heat transfer and pressure drop play a significant role in te PCHE sizing or design process. Altoug te models/correlations for designing commercial PCHEs are proprietary to vendors and ence are not available to te public, extensive studies ave been conducted to develop termal-ydraulic models for designing a variety of PCHEs over te last decade. Kim [11] conducted bot numerical and experimental studies for wavy-cannel PCHEs under ig-pressure elium to elium conditions and developed eat transfer and pressure drop models for wavy-pches wit different cannel pitc angles and cannel diameters. Figure 1 sows individual cannels are etced in a wavy configuration. Te geometry of te wavy cannel is determined by te cannel pitc angle and pitc lengt in te flow direction as indicated in te figure. Figure 1. Wavy-cannel Geometry. 7624 7623

Kim [11] developed correlations for te Fanning friction factor and Nusselt number for te wavy flow cannels tat are Reynolds number dependent as 0.71258 f Re 15.78 0.06677 Re (1) 0.86054 Nu 4.089 0.0083 Re (2) were 300 Re 2, 500, te calculations of Reynolds number are based on te ydraulic diameter of semicircular cannels. Following te termal design metod described in Bartel et al. [13], te size of a wavy-cannel PCHE for elium to elium eat transfer is determined and te results are listed in Table I. Note tat mass flow rates on bot te cold and ot sides of te PCHE are identical. Table I. Design results of te wavy-cannel PCHE Variables Results Variables Results Termal power, kw 13 Total number of plate 16 Hot-side inlet temperature, ºC 800 Plate tickness, mm 1.6 Cold-side inlet temperature, ºC 350 Flow lengt, m 0.203 Hot-side outlet temperature, ºC 462 Hot-side Nusselt number 9.2 Cold-side outlet temperature, ºC 688 Cold-side Nusselt number 9.6 LMTD, ºC 112 Hot-side eat transfer coefficient, W/m 2 -K 2,473 Hot-side inlet pressure, MPa 2 Cold-side eat transfer coefficient, W/m 2 -K 2,353 Cold-side inlet pressure, MPa 2 Overall eat transfer coefficient, W/m 2 -K 1,206 Mass flow rate, kg/ 26.45 Hot-side pressure drop, kpa 2.3 Hot-side Reynolds number 1,753 Cold-side pressure drop, kpa 2.0 Cold-side Reynolds number 1,915 Plate material Alloy 617 3. FABRICATION TECHNIQUES AND DESIGN ASPECTS Following te design of te wavy-cannel PCHE, te eat excanger as been fabricated using 1/16-inc (1.6-mm) tick Alloy 617 plates. A total of eigt ot plates and eigt cold plates are diffusion bonded togeter to form a metal block wit 11 wavy cannels in eac of te plates. Figure 2 sows te geometrical information for bot te ot-side and cold-side plates of te PCHE. Te cross section of te fluid passages is approximately semicircular wit a diameter of 2 mm and a pitc of 2.5 mm in te spanwise direction. Te sape of te flow passage is wavy, and te angle between te flow direction and te edge of te block is 15 degree, as sown in Figure 2. Te PCHE is designed in suc way tat eac side can witstand te maximum design pressure of 3 MPa in te HTHF. Te flow passages in eac plate of te PCHE are made by applying a potocemical etcing tecnique, wic uses strong cemical etcants to remove te selected area on te surface of te plates. Four 20.7- mm diameter troug oles and four 6.35-mm diameter small troug oles are also made on eac of te 16 plates during te cemical etcing process. Figure 3 sows an exploded view and a scematic of te entire PCHE. Two 12.7-mm tick plates, one on te top of te eat excanger block and te oter at te bottom, are also applied to provide an added strengt to te PCHE block. Four out of eigt oles are used to direct te flow into and out of te cannels. At te topmost plate, te four larger oles are connected to four eaders. Te oter four smaller oles are used for alignment during te diffusion bonding process. Four pins are inserted into te four smaller oles before diffusion bonding to prevent te plates from sliding wen adding large load on to te PCHE block to facilitate te diffusion bonding process. Te 7625 7624

dimensions of te PCHE block sown in Figure 3 are 13.35 inces (339.1 mm) in lengt by 4.96 inces (126 mm) in widt by 2 inces (50.8 mm) in eigt. Te ticknesses of bot te topmost plate and bottommost plate are 0.5 inc (12.7 mm). Te topmost plate provides a strong base for joining te four eaders. Te eaders are made from 1 inc NPS Alloy 800H pipes since te piping size in te test facility is 1 inc NPS (corresponding to a pipe scedule of 160). Figure 2. 2-D Drawings of te Wavy-cannel PCHE Hot-side Plate and Cold-side Plate. Figure 3. An Exploded View and a Scematic of te entire PCHE. 7626 7625

Once te potocemical etcing process is completed, all te ot-side and cold-side plates are stacked alternately togeter, and put into a furnace to be diffusion bonded. Diffusion bonding is a solid-state joining tecnique, creating a monolitic joint at te atomic level [14]. Before diffusion bonding, it sould be considered prudently as to ow to connect te four eaders to te PCHE block. Two approaces are discussed ere: one is to perform te diffusion bonding after welding te four eaders on to te topmost plate, wile te oter is to bond te plates togeter wile simultaneously brazing te tubes on to te topmost plate. Figure 4 sows te scematic of tese two approaces to join te four eaders onto te PCHE bonded block. (a) Diffusion bonding after welding te four eaders on to te topmost plate (b) Diffusion bonding te plates togeter wile brazing te tubes on to te topmost plate Figure 4. Scematic of Two Approaces to Join te Four Headers onto te PCHE Block. Te most common way to connect four eaders to te PCHE block is to weld four eaders on to te topmost plate after all te plates are diffusion bonded. Tese eaders could be welded on to te diffusion bonded blocks by te Gas-Tungsten Arc Welding (GTAW) tecnique or oter welding tecniques wit Alloy 617 as te filler material. In tis process, damage may be caused to te PCHE bonded block due to te large termal stress from te welding process. To reduce any potential termal gradients and localized termal stresses, four sort tubes (see Figure 4) are welded on te topmost plate before diffusion bonding and oter connecting tubes are welded on tese four sort tubes. Te four sort tubes provide a distance between te welding locations and te PCHE block, ence reducing te termal gradients in te bonded block. Welding tubes onto te topmost plate before diffusion bonding can relieve te local stress and it does not negatively affect te welding quality. However, it is callenging and expensive to weld te tubes from te bonding surface side because te bottom surface of te topmost plate could deflect due to te large termal gradient from te welding. Terefore, grinding te bottom surface back to its original flat state is required before bonding. In addition, a supporting plate as to accommodate te weld fillet radius and tus puses te effective loading area away from te tubes. It migt not be an issue, but taken to te extreme, it may cause cannel crosstalk due to te unloaded area. Brazing of four eaders onto te PCHE bonded block was applied in te current study since it is an isotermal process and subsequently would not induce large stress on te diffusion bonds. Diffusion bonding te plates togeter wile simultaneously brazing te tubes on to te topmost plate protects te integrity of te diffusion bonded block and brazed joints. However, te aluminum and titanium contents of Alloy 617 could cause incomplete wetting. Te brazing parts sould be plated wit nickel before te brazing process to prevent incomplete wetting. In te present study, te second metod, i.e., brazing four eader on te PCHE bonded block, is adopted. 7627 7626

Te fabrication of te reduced-scale PCHE was completed. Figure 5 sows a picture of te diffusion bonded stack and cannel inlets on te plates. Troug one of te eaders, it can be seen tat te inlet cannels were lined and te inlet recession surface is smooted. Figure 5. Potograps of te Diffusion Bonded Stack and Cannel Inlets on Plates. Te leakage tests for te entire eat excanger were conducted per ASTM elium leak test standard E493 by using a elium leak detector [15]. Te diffusion bonded stack passed te leak test initially, but tere was a leak at a brazed joint (at a leakage at about 6 10-9 l/sec) detected. Re-brazing was performed and te leak was fixed. In summary, a wavy-cannel PCHE is designed for a elium-elium working fluid combination and possesses a 13-kW nominal termal load. Te design pressure and temperature are 3 MPa and 800 ºC, respectively. Te PCHE block is made of Alloy 617 material and four eaders are made from Alloy 800H pipes. Te PCHE core dimension is 13.35 4.96 2 (339.1 126 50.8 mm 3 ) wit eigt plates on eac of te ot and cold sides, consisting of 11 cemically-etced wavy cannels on eac plate. Te actual assembly wit four eaders is sown in Figure 6. 4. PCHE PERFORMANCE TESTING 4.1. Hig-temperature Helium Test Facility Figure 6. PCHE wit Four Headers Assembled. 7628 7627

Experiments were performed to examine te termal-ydraulic performance of te reduced-scale wavycannel PCHE under steady-state conditions. Te experimental study capitalized on te HTHF at Te Oio State University. Figure 7 sows te layout of te ig-temperature elium test facility. Te HTHF was constructed to facilitate termal-ydraulic performance testing of eat excangers at temperatures and pressures up to 800 C and 3 MPa, respectively [2]. Te HTHF consists of pre-eater and main-eater to eat elium to ig temperatures, a gas booster to boost te elium pressure to overcome te pressure drop, a cooler to cool elium down to low temperatures before returning to te gas booster, piping, valves, and various instruments. Pre-eater and main-eater are identical electric eaters wit te same maximum eating capacity of 23 kw. Tree eating elements, eac aving a maximum capacity of approximate 6.7 kw, are embedded on te inner surface of te ceramic fiber insulation and are virtually free-radiating. Te maximum element temperature is 1,300 ºC. Te gas booster installed in te HTHF is a single-stage double-acting air-drive booster. Te elium flow in te system as periodical fluctuations due to te reciprocating action of te booster. Terefore, an inline air-drive pressure regulator valve, in addition to a elium surge tank, was installed downstream of te booster to damp te flow oscillations in te system. In addition, all te measurement sensors ave been calibrated against standards traceable to te National Institute of Standards and Tecnology (NIST). Two straigt-cannel PCHEs made of Alloy 617 and Alloy 800H were tested in tis facility [2]. Figure 7. Layout of te Hig-temperature Helium Test Facility [16]. Figure 8 is a scematic sowing te elium flow pat for eat excanger testing, wen one of te two straigt-cannel PCHEs is replaced by te reduced-scale wavy-cannel PCHE (i.e., te PCHE to be tested). Wen te test PCHE operates at te nominal steady-state condition, te inlet and outlet temperatures of tese two PCHEs are also sown in Figure 8. Te mass flow rate under te nominal steady-state operation is 26.5 kg/. Te termal duties of te pre-eater and main-eater are 6.5 and 4.3 kw, respectively. Figure 8. Scematic of te HTHF System Design wit Heat Excangers Included. 7629 7628

4.2 Experimental Data Reduction Metod 4.2.1 Pressure drop contributions Te test PCHE was installed in te HTHF for te determination of its pressure drop and eat transfer caracteristics. Te first step is to determine te flow friction caracteristics, i.e., te Fanning friction factor. Te approac described by Cen [17] was adopted for tis determination. Figure 9 displays te pressure drop measurement locations on one side of te PCHE. Two differential pressure transducers were installed on eac side of te test PCHE to measure te pressure drops across te PCHE. Te pressure drop associated wit te eat excanger is comprised of two major contributions: (1) pressure drop across te eat excanger core and (2) pressure drop troug fluid distribution devices suc as inlet/outlet flow distribution eaders [3]. Tee Piping Elbow Elbow Piping Tee Flange Flange x y i o Figure 9. PCHE Set-up and Illustration of Pressure Drop Contributions. It can be seen from Figure 9 tat te total pressure drop measured on eac side includes several contributors: pressure loss associated wit te eat excanger core; pressure loss associated wit inlet/outlet flow entrance region; additional pressure loss at te straigt-cannel regions (i.e., regions between surfaces i and x, and y and o as sown in Figure 9) at te inlet/outlet of te excanger; and pressure loss associated wit te piping, fittings (tees and elbows), and flanges between te differential pressure taps. Terefore, factors tat cause pressure drop in te current PCHE are: (1) flow frictional losses in te excanger core; (2) flow momentum rate cange; (3) pressure drop associated wit sudden contraction at te cannel inlets; (4) pressure cange associated wit sudden expansion at te cannel exits; (5) straigt-cannel inlet and outlet regions; and (6) pressure drop in piping, fittings and flanges. To determine te friction caracteristics of te PCHE core, it is necessary to exclude te pressure drop contributions from te experimentally measured pressure drop values tat are not associated wit te eat excanger core friction. Oter pressure drops not related to te eat excanger core pressure drop are subtracted wit te elp of available empirical correlations in te literature. Te experimental Fanning friction factor is ten estimated from te core pressure drop for te wavy cannels of te PCHE. For te core frictional caracteristics, ot-side and cold-side pressures are separately obtained under isotermal and elevated-temperature test conditions. Te assumptions made for pressure drop analysis are as follows: (1) te incoming flow is steady; (2) te fluid is uniformly distributed into eac of te cannel from te eaders; (3) te cannel geometry is identical for all cannels on eac of te cold and ot sides; (4) gravity effect is neglected since te eat excanger is orizontally oriented; and (5) te cross sections of te flow cannel passages are exactly semicircular. Te total measure pressure drop is te sum of all te pressure drop contributions and is expressed as 7630 7629

2 G 2 2 p 1 K 1 K i measure c e 2 i o 2 G 2 i 1 4f l 1 i 2 d i o m p p p p. straigtcannel regions fittings piping flanges (3) Te variables are referred to te Nomenclature. Fanning friction factor, f, te only unknown in Eq. (3), can be determined from te experimental data. 4.2.2 Heat transfer coefficient In te present study, no local internal fluid and wall temperature measurements are available. Terefore, local eat transfer coefficient and mean convective eat transfer on eiter side of te eat excanger cannot be obtained directly. From te PCHE fabrication aspects, te ot-side eat transfer area, A, and s, te cold-side eat transfer area, A, are te same, i.e., A A A. Te overall termal resistance sc, s, sc, s can be expressed as 1 1 1 R. (4) w UA A A s s, c s, c From References [8, 11], te eat transfer model is an only function of te Reynolds number. Te eat transfer coefficient for eiter side can be calculated from te Nusselt number tat can be expressed as a Nu c Re, (5) were a and c are constants. Terefore, te local eat transfer coefficient is given by a Nu c Re, (6) d d were is te termal conductivity of te fluid and d is te cannel ydraulic diameter. Substituting Eqs. (5) and (6) into Eq. (4), Eq. (4) can be written as 1 1 1 RA w s, (7) a a Ud cre cre d c c were U, Re, R, and can be obtained from te experiments; A and d are geometrical parameters. w Two unknown constants a and c can be solved by using nonlinear regression metod to minimize te residual S as: S N 1 RA 1 1 w s, (8) a a j 1 Ud d j cre cre j, j, cj, cj, 2 7631 7630

were N is te total number of te available experimental data points. Once a and c are determined, te Nusselt number correlation (i.e., Eq. (5)) is confirmed. Te eat transfer coefficients for bot sides can be calculated separately from te Nusselt numbers. In addition, te average wall temperature can be obtained via te eat transfer coefficients on bot te ot and cold sides. To determine te overall eat transfer coefficient of te reduced-scale wavy-cannel PCHE, it is necessary to obtain te effective eat transfer area and consider te eat transfer contributions inside te eat excanger. Figure 10 is a scematic of te effective eat transfer area and eat conduction between te two adjacent ports. Te eat transferred from te ot side to te cold side consists of several contributions: (1) eat conduction from port #1 to port #2 and from port #3 to port #4; (2) eat transfer in te crossflow configurations in X and Y regions; (3) eat transfer in te countercurrent flow region Z; and (4) eat loss to te surroundings. Four termocouples were installed at locations 1, 2, 3, and 4 (see Figure 10) in te HTHF to measure te inlet and outlet temperatures of te eat excanger. Tree experimental data points are used to examine te eat conduction from port #1 to port #2 and from port #3 to port #4, i.e., contribution #1 mentioned above. Te average ratio of te total eat conduction to te total eat transferred from te ot side to te cold side is about 0.4%, wereas te overall eat transfer coefficient is 1.1% iger tan tat witout considering tis eat conduction. Canges in te four terminal temperatures result in te LMTD decreasing by 1.1%. A smaller LMTD gives a larger overall eat transfer coefficient wen te eat conduction (contribution #1) between te ports is neglected. Note tat te eat transfer area summation of regions X, Y, and Z, sowing in Figure 10, is te effective eat transfer area tat is adopted in te current study. Figure 10. Scematic of te Effective Heat Transfer Area and Heat Conduction Pat (1, 2, 3, and 4 Represent te Hot-side and Cold-side Ports or Plena). 4.2.3 Uncertainty analysis An uncertainty analysis, considering error propagation, as been performed using te root-sum-square metod. Te uncertainty in te Fanning friction factor is based on te simplified form of te eat excanger core friction pressure drop given by f p d 2 l A m 2 c 2. (9) Te uncertainty in Fanning friction factor can be expressed as 7632 7631

2 2 2 2 2 2 f p d A m c l 2 2. f p d m A l c (10) Te eat transfer model is obtained by using te nonlinear regression metod. Te perturbation metod is used to determine te uncertainties in te fitted eat transfer correlation [18], i.e., uncertainties in constants a and c in Eq. (5). Te uncertainties in Re, Re c,,, A, d and R in Eq. (8) can be c c w determined from te experimental data. One of te seven variables is canged to its upper bound and te corresponding values of a and c 1 1 are determined. Tese values are compared wit te original least squares values to determine a and c. Te same procedure is repeated for its low bound value in te 1 1 same variable, a and c are ten obtained. Tese steps are repeated for te remaining variables. 2 2 Tere are fourteen regression runs required to compute all individual relative errors in constants a and c. Finally, te uncertainties in a and c are calculated separately by taking te root-mean-square metod as: a c were n is te number of te regression runs. 4.3 Preliminary Experimental Data Analysis 1 n 1 n n i1 n i1 a c 2 i 2 i, (11) Preliminary performance testing of te wavy-cannel PCHE as been carried out. Te test loop was carged wit elium to a desired pressure from te elium gas cylinder wit a purity rating of 99.9999% after te test loop was vacuumed to te desired vacuum pressure of -14 psig by using a vacuum pump. Te cooler was turned on during all te tests to avoid ig-temperature elium going troug te gas booster and damaging it. Te elium flow in te test loop was driven by te gas booster. Before data processing, te mass flow rates were cecked for all experimental runs and te oscillations of te mass flow rates recorded by two Venturi flow meters installed in te HTHF were less tan 1% of teir respective mean values. 4.3.1 Pressure drop caracteristics Te experimentally obtained isotermal Fanning friction factors were compared wit tose obtained by Kim s friction factor correlations [11] tat were used in te PCHE termal design. Figures 11 and 12 sow te plots of te experimental Fanning friction factor compared wit te results obtained from Kim s correlations for bot te ot and cold sides of te PCHE. As can be seen from te figures, te experimentally obtained Fanning friction factors follow te trend establised in Kim s model well, wile discrepancies between te experimental data and te correlations are presented mainly due to te cannel differences. Kim used wavy cannels wit sarp turns at all of te bends, wereas a radius of curvature was induced at eac bend in all cannels of te test PCHE. Te cross section is not exactly a semicircular sape in te test PCHE, wile Kim s numerical model is for wavy cannels wit a perfect semicircular cross section. Compared to te experimental data, te largest differences wit Kim s model are 19.8% and 32.1% for te ot side and cold side, respectively. Te discrepancies between te experimental data and 7633 7632

numerical results are presented since te pressure drop in te laminar flow region is strongly dependent on te geometry and size of te flow cannel. Te Fanning friction factors on te ot and cold sides are different, wic is attributed to te flow cannel geometry as well. Te cannels geometric parameters on te ot and cold sides are not exactly identical due to manufacturing imperfections and tolerance. For bot te ot and cold sides, te uncertainties are 17.2% and 23.4% for te Fanning friction factors reduced from te data at te smallest Reynolds number and te largest Reynolds number, respectively. Figure 11. Fanning Friction Factor on te Hot Side. 4.3.2 Heat transfer caracteristics Figure 12. Fanning Friction Factor on te Cold Side. Equation (12) is te fitted eat transfer correlation wit uncertainties in te constants identified, wic is obtained by following te metod described in Sections 4.2.2 and 4.2.3. Te fitted experimental correlation, as sown in Figure 13, indicates tat te Nusselt number increases gradually wit te increasing Reynolds number. Te increase of te Nusselt number wit increasing te Reynolds number occurs due to te velocity increase, resulting in a more turbulent fluid flow. As te Reynolds number decreases, te turbulence in te flow tends to decrease and eventually diminis. As a result, te eat transfer coefficient decreases. Te open-stared line as sown in Figure 13 presents te upper limit of te 7634 7633

fitted Nusselt number, wic can be obtained by canging constants a and c to teir upper bounds. Wen a and c are given to teir lower bounds, te lower limit of te fitted Nusselt number is obtained, as sown in open-triangled line in Figure 13. Figure 13 also sows te comparison between te Nusselt numbers from te experiments and from Kim s model (open-circled line). Results obtained from Kim s model sow some discrepancies wit te current correlation. Kim s model presents a slower Nusselt number increase rate wit te increase of Reynolds number tan te current correlation. Larger deviations can be seen in te low Reynolds number region were te current correlation gives smaller Nusselt number values tan te model developed by Kim. As te Reynolds number increases, te differences between te two results decrease from 31.6% to 0.4%. Nusselt number is strongly dependent on te cannel geometry and boundary conditions in laminar flow regime. As described in Section 4.3.1, te cannel geometry used in te present study is different from tat used in Kim s simulation. In addition, te boundary condition of Kim s modeling is constant wall eat flux wit constant circumferential wall temperature. However, te actual testing boundary condition in our experiments is muc more complicated and may not reflect te condition used in te Kim s model. Nu c Re a c 0. 028899 7. 393333 10 4 (12) a 0. 75508 1. 95556 10 3 5. CONCLUSIONS Figure 13. Nusselt Number for bot te Cold and Hot Sides. In tis study, one reduced-scale PCHE was fabricated using Alloy 617 plates for te eat excanger core and Alloy 800H pipes for te eat excanger eaders. Te detailed fabrication tecniques are presented. Te termal-ydraulic performance of te PCHE was investigated experimentally in te ig-temperature elium test facility located at Te Oio State University. Comparisons between te obtained experimental data and available empirical correlations indicated tat bot te ot-side and cold-side friction caracteristics of te PCHE follow te trend in te empirical model well. Heat transfer caracteristics obtained from te experimental result sow some discrepancies mainly due to te geometric differences. Larger deviations appear in te low Reynolds number region. Finally, te constants in te correlation for te convective eat transfer coefficient model for te wavy cannels are proposed based on te experimental data. 7635 7634

NOMENCLATURE a constant in te eat transfer correlation, Eq. (5) cannel cross-section area A c A s eat transfer area c constant in te eat transfer correlation, Eq. (5) cannel ydraulic diameter d f G K c Fanning friction factor mass flux eat transfer coefficient contraction loss coefficient K exit loss coefficient e l flow lengt m mass flow rate n number of regression runs N number of experimental data points Nu Nusselt number p pressure drop Q eat conduction rate in Figure 10 R w wall termal resistance Re Reynolds number S residual T temperature U overall eat transfer coefficient x, y wavy cannel inlet and outlet surfaces X,Y,Z effective eat transfer regions in te test PCHE Greek symbols elium density uncertainty wavy pitc angle ratio of eat excanger core minimum free-free area to frontal area elium termal conductivity Subscripts c cold side ot side i inlet t j j experiment, index m mean value o outlet ACKNOWLEDGEMENTS 7636 7635

Tis researc is being performed using funding received from te U.S. Department of Energy Office of Nuclear Energy's Nuclear Energy University Programs. Te assistance provided by Mr. Kevin Wegman of Te Oio State University is appreciated. REFERENCES 1. P. Sabarwall, E.S. Kim, M. McKellar, and N. Anderson, Process Heat Excanger Options for te Advanced Hig Temperature Reactor, INL/EXT-11-21584, Idao National Laboratory (2011). 2. S.K. Mylavarapu, Design, Fabrication, Performance Testing, and Modeling of Diffusion Bonded Compact Heat Excangers in a Hig-temperature Helium Test Facility, Doctoral dissertation, Te Oio State University (2011). 3. R.K. Sa and D.P. Sekulic, Fundamentals of Heat Excanger Design, Jon Wiley & Sons (2003). 4. Heatric, ttp://www.eatric.com/typical_caracteristics.tml, accessed on April 15 t (2015). 5. V. Dostal, A Super Critical Carbon Dioxide Cycle for Next Generation Nuclear Reactors, Master tesis, Massacusetts Institute of Tecnology (2004). 6. K. Gezelius, Design of Compact Intermediate Heat Excangers for Gas Cooled Fast Reactors, Master tesis, Massacusetts Institute of Tecnology (2004). 7. M. Cen, I.H. Kim, X. Sun, R.N. Cristensen, V.P. Utgikar, and P. Sabarwall, Transient Analysis of an FHR Coupled to a Helium Brayton Power Cycle, Progress in Nuclear Energy, 83, pp. 283-293 (2015). 8. K. Nikitin, Y. Kato, and L. Ngo, Printed Circuit Heat Excanger Termal-ydraulic Performance in Supercritical CO 2 Experimental Loop, International Journal of Refrigeration, 29(5), pp. 807-814 (2006). 9. T.L. Ngo, Y. Kato, K. Nikitin, and T. Isizuka, Heat Transfer and Pressure Drop Correlations of Microcannel Heat Excanger wit S-saped and Zigzag Fins for Carbon Dioxide Cycles, Experimental Termal and Fluid Science, 32, pp. 560-570 (2007). 10. E.S. Kim, C. O, and S. Serman, Simplified Optimum Sizing and Cost Analysis for Compact Heat Excanger in VHTR, Nuclear Engineering and Design, 238(10), pp. 2635-2647 (2008). 11. I.H. Kim, Experimental and Numerical Investigations of Termal-ydraulic Caracteristics for te Design of a Printed Circuit Heat Excanger (PCHE) in HTGRs, Doctoral dissertation, Korea Advanced Institute of Science and Tecnology (2012). 12. S.H. Yoon, H.C. No, and G.B. Kang, Assessment of Straigt, Zigzag, S-sape, and Airfoil PCHEs for Intermediate Heat Excangers of HTGRs and SFRs, Nuclear Engineering and Design, 270, pp. 334-343 (2014). 13. N. Bartel, M. Cen, V.P. Utgikar, X. Sun, I.H. Kim, R.N. Cristensen, and P. Sabarwall, Comparative Analysis of Compact Heat Excangers for Application as te Intermediate Heat Excanger for Advanced Nuclear Reactors, Annals of Nuclear Engineering, 81, pp. 143-149 (2015). 14. B. Derby and E.R. Wallac, Teoretical Model for Diffusion Bonding, Metal Science, 16(1), pp. 49-56 (1982). 15. Standard Test Metods for Leaks Using te Mass Spectrometer Leak Detector in te Inside-out Testing Mode, ASTM E493-06, ASTM International (2006). 16. S.K. Mylavarapu, X. Sun, R.E. Glosup, R.N. Cristensen, and M.W. Patterson, Termal-ydraulic Performance Testing of Printed Circuit Heat Excangers in a Hig-temperature Helium Test Facility, Applied Termal Engineering, 65, pp. 605-614 (2014). 17. M. Cen, Design, Fabrication, Testing, and Modeling of a Hig-temperature Printed Circuit Heat Excanger, Master tesis, Te Oio State University (2015). 18. H.F. Kartabil and R.N. Cristensen, An Improved Sceme for Determining Heat Transfer Correlations from Heat Excanger Regression Models wit Tree Unknowns, Experimental Termal and Fluid Science, 6, pp. 808-819 (1992). 7637 7636