Ishibashi, Akira

Lee, Sangmu

Matsuda, Takuya

Shiba, Hirokuni

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2010Heat Transfer and Pressure Drop Characteristics ofCross Stair Slit FinTakuya MatsudaMitsubishi Electric CorporationAkira IshibashiMitsubishi Electric CorporationSangmu LeeMitsubishi Electric CorporationHirokuni ShibaMitsubishi Electric CorporationFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlMatsuda, Takuya; Ishibashi, Akira; Lee, Sangmu; and Shiba, Hirokuni, "Heat Transfer and Pressure Drop Characteristics of Cross StairSlit Fin" (2010). International Refrigeration and Air Conditioning Conference. Paper 1070.http://docs.lib.purdue.edu/iracc/1070  2261, Page 1  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  Heat Transfer and Pressure Drop Characteristics of Cross Stair Slit Fin Takuya Matsuda1*, Akira Ishibashi1, Sangmu Lee1, Hirokuni Shiba2  1Living Environment Systems Laboratory, Mitsubishi Electric Corporation, 3-18-1, Oshika, Suruga-ku, Shizuoka-city, 422-8528, Japan Tel.: +81-54-287-3053, Fax: +81-54-287-3056, E-mail: Matsuda.Takuya@ds.MitsubishiElectric.co.jp  2Shizuoka Works, Mitsubishi Electric Corporation, 3-18-1, Oshika, Suruga-ku, Shizuoka-city, 422-8528, Japan    ABSTRACT  The new type of slit fin “Cross Stair Slit Fin” for the outdoor fin-tube heat exchanger has been developed to enhance heat transfer on dry and wet surface condition and restrain heat transfer performance degradation on frost surface condition. The shape of cross stair slit fin was optimized using relationships between air side heat transfer and pressure drop by CFD. The air side heat transfer and pressure drop of 3 different fin shapes (ring, conventional slit, cross stair slit) were measured on dry, wet and frost surface condition. The air side heat transfer of cross stair slit fin on dry and wet surface condition was 10% larger than ring fin and 10% smaller than conventional slit fin. Also, heat transfer performance of cross stair slit fin on frost surface condition was the same as ring fin, and 40  larger than conventional slit fin.  1. INTRODUCTION  The slit fin has been studied in order to improve the performance of fin-and-tube heat exchanger for air conditioners. However, in case of the outdoor fin-tube heat exchanger, frost grows on the slit. Therefore air side pressure drop increases compared with flat fin without slit on the frost condition. And the heat transfer performance of slit fin is decreased as air flow rate decreases. In this study, new type of slit fin “Cross Stair Slit Fin” has been developed to enhance heat transfer on dry and wet surface condition and prevent heat transfer performance degradation on frost condition. The shape parameters of cross stair slit fin which influences on air side heat transfer and pressure drop were optimized by CFD. The air side heat transfer and pressure drop of 3 different fin shapes (ring, conventional slit, cross stair slit) were measured on dry, wet and frost surface.  2. CONCEPT AND PERFORMANCE OF CROSS STAIR SLIT FIN  2.1 Concept of Cross Stair Slit Fin The concept of cross stair slit fin is increasing heat transfer on dry and wet surface condition and preventing heat transfer performance degradation on frost condition. If the slits are blocked by the frost growth, the bypass air flow rate can be kept to restrain the increase of pressure drop. Figure 1 shows the shape of cross stair slit fin. The slit1 is divided into two parts and slit2 of the fin center is formed like ‘crossing’. And the height of slit1 H1 is lower than the height of slit2 H2 like a ‘stair’. Therefore the frost is decentralized, and bypass air flow rate is kept. In addition, the decrease of heat transfer performance on frost condition can be prevented. On dry and wet condition the heat transfer increases by dividing the boundary layer, because air passes through the slit.     2261, Page 2  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  BCA AA-A cross-sectionB-B cross-sectionBCC-C cross-sectionFpH1H2Dp LDo((Slit1 Slit2H1 H2 Figure1: Cross stair slit pattern  2.2 Numerical Analysis Evaluation  2.2.1 Analysis condition A representative parameter of cross stair slit fin is the width between two slit1 L. L is equal to the length of slit2. L divided by step pitch Dp is L/Dp. Divided width ratio L/Dp is the dominant parameter to the slit performance. By using CFD (FLUENT), the influence of L/Dp on the air side heat transfer coefficient o and air side pressure drop P were calculated. Figure 2 shows the boundary condition of numerical analysis. An analysis model is three-dimensional non-steady flow. The flow field is laminar. The pressure solution is SIMPLE method, and 1st-order implicit method is used for convective and diffusion term. The inlet boundary condition of the air is 1m/s, 293K (20 C) . The outlet boundary condition is free flow, the fin surface is the wall boundary condition, and temperature conditions of the fin collar is 323K (50 C). The periodic boundary condition is used for the other surface of the analysis area.     Figure 2: Boundary condition  2.2.2 Analysis result Figure 3 shows the relationship between L/Dp and the air side heat transfer coefficient o. Figure 4 shows the relationship between L/Dp and air side pressure drop P. As L/Dp increases, both the air side heat transfer Fp Periodic boundary condition  Periodic boundary condition Wall boundary condition of temperature (323K)  Inlet boundary condition (1m/s, 293K) Outlet boundary condition Air   2261, Page 3  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  coefficient o and pressure drop P increase.  The relationship between the heat transfer coefficient o and the pressure drop P were evaluated in terms of the coefficient of performance (COP) of the heat pump unit. The increase of the heat transfer of the fin means the increase of COP by the decrease of the input power of the compressor, and the increase of the air side pressure drop means the decrease of COP by the increase of the input power of the fan. So o/ P was defined as the performance index of the heat exchanger from the relationship between the input power of the fan and the compressor. Figure 4 shows the influence of the L/Dp on o/ P. When L/Dp is 0.2, o/ P showed the maximum value.   14.214.715.215.716.216.70 0.1 0.2 0.3 0.4 0.5 0.650556065707580PoL/Dp [-]Heat transfer coefficient o [W/(m2 K)]Pressure drop P[Pa]4.5254.5334.5414.5494.5574.5650 0.1 0.2 0.3 0.4 0.5 0.6o/ PL/Dp [-]o/P [W/(m2 KPa)] Figure 3: Influence of L/Dp on o and P                                Figure 4: Influence of L/Dp on o/ P   3. EXPERIMENTAL APPARATUS AND METHOD  The prototype of heat exchanger with cross stair slit pattern picked out by CFD was produced. And, 2 different fin shapes (ring, conventional slit) were produced to compare with the cross stair slit. The air side heat transfer and pressure drop of 3 different fin shapes (ring, conventional slit, cross stair slit) were measured on the condition of dry, wet and frost surface. Experimental apparatus, test heat exchangers and experimental method are explained below.  3.1 Experimental Apparatus Figure 6 shows the schematic view of experimental apparatus. Test heat exchangers are put into the wind tunnel, and the wind tunnel is in the constant temperature and humidity room. The wind tunnel is square shape with cross section area 300  300 mm. Along the air flow, the honeycomb, the thermocouple grid, the dew point meter, pressure taps, a test heat exchanger (Figure 5), and the fan are installed in the wind tunnel. The fluid flowing in the test heat exchanger pipe is water or brine. The temperature of the fluid is measured by the sheathed resistance thermometer (Pt100, ClassA). The mass flow rate of the fluid is measured by the coriolis mass flow meter (Accuracy: 0.2%, R.D.). The airside temperature is measured by the thermocouple grid (Accuracy: 0.1 C). The dew-point temperature is measured by the mirror surface cooling type dew-point meter (Accuracy: 0.2C). The air flow rate is measured by the ultrasonic volume flow meter  (Accuracy: 0.03%, F.S.). The pressure drop is measured by the differential-pressure meter (Accuracy: 0.1Pa)       2261, Page 4  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  300mm245mmWater InletWater OutletAirPlatinum Thermo resistanceT2 T2T1Water InletWater OutletAir Inlet1 HoneycombT11232 Test Heat Exchanger3 FanPTd TdPressure TapsT2Thermocouple GridTd Dew Point MeterAir OutletPPT1 Figure 5: Experimental apparatus                                           Figure 6: Test heat exchanger 3.2 Test Heat Exchanger Table1 shows specifications of test heat exchangers. The specifications of fin patterns are ring, conventional slit and cross stair slit. The specification of ring fin is that tube diameter is 7.94mm, step pitch is 20.4mm, row pitch is 17.7mm, number of row is 3, fin pitch is 1.6mm, fin thickness is 0.09mm, and surface treatment is bare (no-hydrophilic coated). The specification of conventional slit fin and cross stair slit fin are that tube diameter is 7.94mm, step pitch is 20.4mm, row pitch is 17.7mm, number of row is 3, fin pitch is 1.8mm, fin thickness is 0.09mm, and surface treatment is bare (no –hydrophilic coated).   Table 1: Specifications of test heat exchangers  Fin pattern Ring Conventional slit Cross stair slit Tube Diameter: Do[mm] 7.94 7.94 7.94 Step pitch: Dp[mm] 20.4 20.4 20.4 Row Pitch: Lp[mm] 17.7 17.7 17.7 Number of Row: N[-] 3 3 3 Fin pitch: Fp[mm] 1.6 1.8 1.8 Fin Thickness: Tf[mm] 0.09 0.09 0.09 Surface treatment Bare (no-hydrophilic coated)Bare (no-hydrophilic coated)Bare (no-hydrophilic coated)Fin shape DpLpDoFp Tf      2261, Page 5  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  3.3 Experimental Condition and Method Table 2 shows experimental conditions. Air side heat transfer coefficient o and pressure drop P are measured on dry, wet and frost surface condition. On the dry surface condition, the inlet temperature of air is 20 C, the fluid in the tube is water, and the inlet temperature of water is 50 C. By changing the air velocity and the water flow rate are changed, air side heat transfer coefficient o is measured by Wilson-Plot method. On the wet surface condition, the inlet temperature of air is 27 C, the inlet absolute humidity of air is 10.5 g/kg, the fluid in the tube is water, and the inlet temperature of water is 10 C. On the frost surface condition, the inlet temperature of air is 2 C, the inlet absolute humidity of air is 3.7 g/kg, the air velocity is 1.5m/s, the fluid in the tube is brine, the inlet temperature of brine is -5 C, and the brine flow rate is 6 l/m.  Table 2: Experimental condition  Surface State of Fin Dry Wet Frost Inlet Temperature of Air [ C] 20 27 2 Inlet Absolute Humidity of Air [g/kg] (Inlet Relative Humidity[%]) - 10.5 (47) 3.7 (84) Air Velocity[m/s] 1,1.5,2 1,1.5,2 1.5 Fluid Water Water Brine Flow Rate[l/min] 7,9,11,13 7,9,11,13 6 Inlet Temperature of fluid[ C] 50 10 -5  4. EXPERIMENTAL RESULTS  4.1 Dry Surface Condition Figure 7,8 shows the experimental result of the air side heat transfer coefficient o and pressure drop P to the air velocity on dry surface condition. The calculation result of cross stair slit fin is shown in the same figure. The deviation between the calculation and experimental results of the air side heat transfer coefficient o is within 6%. The deviation of the air side pressure drop P is within 10%. When the air velocity is 1.5m/s, the air side heat transfer coefficient of conventional slit fin is highest, and o of cross stair slit fin is higher than ring. The same tendency is shown in case of pressure drop. The air side heat transfer coefficient o of conventional slit fin is 23% larger than ring fin, and pressure drop P is 27% larger. On the other hand, the air side heat transfer coefficient o of cross stair slit fin is 12% larger than ring fin, and pressure drop P is 2% larger.  304050607080901000.5 1 1.5 2 2.5Ring Conventional slitCross stair slit (exp.)Cross stair slit (cal.)Air velocity [m/s]Heat transfer coefficient o [W/(m2 K)] 01020304050600.5 1 1.5 2 2.5Ring Conventional slitCross stair slit (exp.)Cross stair slit (cal.)Air velocity [m/s]Pressure drop P [Pa] Figure 7: Air side heat transfer coefficient o                               Figure 8: Air side pressure drop P (on dry surface condition)                                                       (on dry surface condition)   2261, Page 6  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  4.2 Wet Surface Condition Figure 9,10 shows the experimental result of the air side heat transfer coefficient oh and pressure drop P to the air velocity on wet surface condition. oh is calculated based on air enthalpy. When the air velocity is 1.5m/s, the air side heat transfer coefficient oh of cross stair slit fin is 10% larger than ring fin. But the air side pressure drop P is 7% smaller than ring fin. The increasing ratio of pressure drop Pwet / Pdry is defined as the ratio of the wet pressure drop to dry pressure drop. Figure 11 shows Pwet / Pdry to the air velocity. When the air velocity is 1.5m/s, Pwet / Pdry of cross stair slit fin is 10% smaller than ring fin. Figure 12, 13 shows the photograph of real surface states on the wet condition. The condensed water is hold between fins, because the surface treatment of the both fins are bare (no-hydrophilic coated). The hold area of cross stair slit fin is smaller than that of ring fin. The condensed water is moved to the silts and drained, because the gravity force of the condensed water and the shear stress of the air are larger than the surface tension. In the result, the drainage performance of the cross stair slit fin is improved, and the increase ratio of pressure drop of cross stair slit fin is smaller than ring fin.   0.030.0350.040.0450.050.0550.060.5 1 1.5 2 2.5Ring Cross stair slitAir velocity [m/s]Heat transfer coefficient  oh [kg/(m2 s)] 015304560750.5 1 1.5 2 2.5Ring Cross stair slitPressure drop P [Pa]Air velocity [m/s] Figure 9: Air side heat transfer coefficient oh                            Figure 10: Air side pressure drop P (on wet surface condition)                                                     (on wet surface condition) 1.51.61.71.81.922.10.5 1 1.5 2 2.5Ring Cross stair slitAir velocity [m/s]Pwet/P dry [-]               Figure 11: Increasing ratio of pressure drop Pwet / Pdry     2261, Page 7  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010    Figure 12: The surface state of ring fin                Figure 13: The surface state of cross stair slit fin (on wet surface condition)                                              (on wet surface condition) 4.3 Frost Surface Condition The time changes of air side pressure drop P are shown in Figure14. On the same time condition, the air side pressure drop P of conventional slit fin is highest, and that of cross stair slit fin is approximately the same as ring fin. In order to compare the heat transfer performance on frost surface condition, the amount of frost weight M is calculated below. At first, the frost weight on a time interval ( t  M is given by equation (1). Ga is the air mass flow rate, (Xa1-Xa2) is the absolute humidity difference between the inlet and the outlet. And, the total amount of frost weight M is calculated by equation (2). Here n is total number which the air pressure drop reaches 200Pa.  tXXGM aaa 21          (1) )200(1PniMM           (2)  Figure 15 shows M to each fin shape. The total amount of frost weight of cross stair slit fin is approximately 40% larger than that of conventional slit, and approximately the same as ring fin.  0501001502000 20 40 60 80RingConventional slitCross stair slittime [min]Pressure drop P[Pa]  00.050.10.150.20.25Ring Conventional slit Cross stair slitTotal weight of  frost M [kg]69%95%100%     Figure 14: The time changes of air side pressure drop P                Figure 15: Total amount of frost weight M (on frost surface condition)  2261, Page 8  International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010  5. CONCLUSIONS Cross stair slit fin for the outdoor fin-tube heat exchanger has been developed to increase heat transfer on dry and wet surface condition and restrain heat transfer performance degradation on frost surface condition. The shape of cross stair slit fin was optimized using relationship between air side heat transfer and pressure drop by CFD. The air side heat transfer and pressure drop of test heat exchangers with 3 different fin shapes (ring, conventional slit, cross stair slit) were measured on dry, wet and frost surface conditions. Conclusions are shown as follows.  The air side heat transfer coefficient o of cross slit fin is 12% larger, and pressure drop P is 2% larger compared with ring fin on dry surface condition. Increasing ratio of pressure drop of cross stair slit fin is 10% smaller than ring fin, because the condensed water on the slits is well drained. The heat transfer performance of cross stair slit fin was the same as ring fin and 40% larger than conventional slit fin on frost condition. The deviation between the calculation and experimental results of the air side heat transfer coefficient o of cross stair slit fin is within 6% on the dry condition. The deviation of the air side pressure drop P is within 10%    NOMENCLATURE  COP coefficient of performance of heat pump unit [-] Do tube outer diameter  [mm] Dp step pitch  [mm] Ga air mass flow rate  [kg/s] H1 height of slit1  [mm] H2 height of slit2  [mm] L/Dp divided width ratio  [-] L width between two slit1  [mm] M total weight of frost  [kg] Xa1 inlet absolute humidity of air   [kg/kg] Xa2 outlet absolute humidity of air  [kg/kg] P air side pressure drop  [Pa] o air side heat transfer coefficient  [W/(m2K)] oh air side heat transfer coefficient based on enthalpy [kg/(m2s)]   REFERENCES T. Matsuda, et al. , 2008, Heat Transfer and Pressure Drop Characteristics of Cross Stair Slit Fin, Proc. 2008 JSRAE Annual Conf., p.587-590 A. Ishibashi, et al. , 2003, Heat Transfer and Pressure Drop Characteristics on Slit Finned Tube Heat Exchanger for Outdoor Unit, 40th National Heat Transfer Symposium of Japan, p.491 H. Seshimo, 1990, Doctoral Thesis at Kobe University T. Ohkouchi, et al., 2005, The Heat Transfer and Pressure Drop Characteristics of the Heat Exchanger with Wing Fin, Denso Technical Review, p.96-105  

Heat Transfer and Pressure Drop Characteristics of Cross Stair Slit Fin

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Heat Transfer and Pressure Drop Characteristics of Cross Stair Slit Fin

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