Air-Side Thermal Performance of Micro-Channel Heat Exchangers Under Dehumidifying Conditions

An experimental study for air-side thennal-hydraulic performance of brazed aluminum heat exchangers under dehumidifYing conditions has been performed. For 30 samples of louvered fin heat exchangers with different geometrical parameters, the heat transfer and pressure drop characteristics for wet surface were evaluated. The test was conducted for air-side Reynolds number in the range of 80-300 and tube-side water flow rate of 320kg/h. The dryand wet-bulb temperatures of the inlet air for heat exchangers were 27°C and 19°C, respectively and the inlet water temperature was 6°C. The air-side thennal performance data for cooling and dehumidifYing conditions were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluids unmixed. The test results were reported, compared with those for the dry surface heat exchangers, in terms of sensible j-factor and friction factor f, as functions of Reynolds number based on louver pitch. The correlations for j and f factors are developed within rms errors of 16.9 and 13.6 %, respectively. NOMENCLATURE Ac : Minimum free-flow area for air side, m ReLP : Reynolds number based on louver pitch At : Fin surface area, m T : Temperature, K Afr : Frontal area, m Td : Tube depth, m A ow :Total air-side surface area, m T : Temperature difference, K At : External tube surface area, m Um.Aow : Overall thermal conductance, W IK Aw : Tube wall area, m Vc :Maximum air velocity, m/s Cp : Specific heat, JlkgK Cr : Capacity ratio Greek letters Dh : Hydraulic diameter, m a : Louver angle, deg f : Fanning friction factor r :Aspect ratio of tube hole FP : Fin pitch, m 0;: Fin thickness, m Fd : Flow depth, m ~. :Tube wall thickness, m H : Fin height, m e : Effectiveness h : Heat transfer coefficient, W/ mK T/jiv : Fin efficiency how : Total heat transfer coefficient, W/ mK T/mv :Surface effectiveness ho : Sensible heat transfer coefficient, W/ mK p : Density, kg/m j : Colburnj-fuctor (Nu/Re W ) Pm : Mean average air density, kg/m 3 k :Thermal conductivity, W/mK ka/ :Thermal conductivity of tube wall, W/mK (Y :Contraction mtio of the fin array (A/A,;-) Kc : Abrupt contraction coefficient Ke : Abrupt expansion coefficient Subscripts I : Fin length, m ave : Average value La : Louver angle, deg 1 : Inlet for air side 2 : Outlet for air side Ll : Louver length, m i :Waterside Lp : Louver pitch, m I :Louver m :Mass flow rate, kg/s : Maximum value m* :Refer Eq(ll), mmax min : Minimum value Nu : Nusselt number (hD!k) :Air side 0 p : Pressure drop, Pa :Tube Pr : Prandtl number (via) w : Tube wall or water Q : Heat transfer rate, W ow : Wet surface Re : Reynolds number Eighth International Refrigeration Conference at 119 Purdue University, West Lafayette, IN, USAJuly 25-28,2000 INTRODUCTION Heat exchangers play an important role in the energy efficiency and physical size of the refrigeration and air conditioning system. Heat exchangers in refrigeration and air conditioning applications are classified as a condenser and an evaporator and fmned round tube heat exchangers are used extensively. Improving the technology of heat exchanger performance, particularly air-side performance, has attracted many investigators [1-3]. Since the surface of the evaporator is subject to condensation of moisture contained in air in cooling mode operation, the design of airside configuration of the heat exchanger requires consideration of heat and mass transfer simultaneously. Research on the air-side thermal performance of finned tube heat exchangers under wet condition has been performed by several investigators [4-18]. McQuiston [4-6] studied heat and mass transfer on wet coils and developed correlations for the heat transfer coefficients and pressure drops. Threlkeld (7], McQuiston [8], and Wu and Bong (9] provided the fm efficiency for the fully wet surface heat exchangers. Wu and Bong also presented the overall fm efficiency for the partially wet surface, and reported that only when the fm is partially wet the overall fin efficiency depends significantly on the relative humidity. Hill and Jeter [10] developed a linear sub-grid model for the air conditioner's cooling and dehumidifying coil which is an evaporator. They showed the single-pass, cross flow arrangement of the model was adequate to model counter cross flow heat exchangers. Mirth et al. [11] investigated performance analysis of the cooling coil based on AR1 Standard [12] and Hu et aL (13] presented the effect of the shape of the condensation water on the fin surface on thermal performance characteristics. Youn et al. [14] and Domanski and Didion (15] proposed a model to analyze the performance for various air conditioning evaporators based on tube-by-tube method, and reported that the sensible heat transfer coefficient for wet surface was larger than that for dry surface. On the other hand, Wang et al. [16] reported the heat transfer coefficient for wet surface is smaller than that of dry surface based on a study on the thermal-hydraulic performance of the dehumidifYing coil. Chuah et al. [17] investigated dehumidifYing performance of chilled water coils with variation of water flow rate. Kim and Jacobi [18] investigated condensation accumulation effects on air-side heat transfer and pressure drop characteristics for plain and slit fms and round tube heat exchangers. However, there are few data on the air-side performance for louvered fin brazed aluminum heat exchangers with dehumidification. Webb and Jung [19] carried out one study for applying the brazed aluminum heat exchangers to the residential air conditioner, and showed heat transfer rate of the brazed aluminum heat exchanger was 50% higher than that of conventional heat exchanger. They reported drainage of condensation water on the heat exchanger surface could be removed well and it could be used as an evaporator for the residential air conditioning system. Chiou et al. [20] investigated thermal performance of serpentine type automotive evaporator with flat tube and louvered fins. They reported that the sensible heat transfer coefficient for wet surface was larger than that for dry surface. However, there is no published data in the open literature for the correlations of j and f factors of brazed aluminum heat exchangers under wet condition. This study presents the heat transfer and pressure drop behaviors of brazed aluminum heat exchangers under wet surface condition. A series of tests are conducted for the heat exchangers with several different air-side configurations such as louver angle, flow depth, and fin density. Test results are compared with those for dry surface, and sensible j-factor and friction factor fare reported as functions of Reynolds number based on louver pitch, and correlations for the j and ffactors are extracted from the test data. EXPERIMENTS Test apparatus Figure 1 shows a schematic diagram of the test apparatus used in the study. It consists of a suction type wind tunnel, heat transfer fluid (water) circulation and control units, and data acquisition system and is situated in a constant temperature and humidity chamber. The air inlet condition of the heat exchanger can be maintained by controlling the chamber temperature and humidity. The air inlet and outlet dry and wet bulb temperatures for the heat exchanger are measured associated with sampling units. The air-side pressure drop through the heat exchanger is measured using a differential pressure transducer and air flow rate is measured using nozzle pressure difference. The heat transfer fluid circulation and control units can maintain the inlet condition of water-side by regulating water flow rate and inlet temperature. The uncertainty of heat transfer rate for the test apparatus is within 3% since accuracy of Eighth International Refrigeration Conference at 120 Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 temperature measurement is ±0.1 oc and accuracy of air and water flow rates are ±1% and ±2%, respectively. Test heat exchangers Figures 2 and 3 indicate geometrical configuration and terminology of the test heat exchanger. The heat exchangers are louvered fin and micro-channel heat exchangers; 30 heat exchanger samples are used for the test. Table 1 shows simple specification of the test heat exchangers. Fin pitches are 1.2, and 1.4 rom for all cases, plus 1.0 rom for heat exchangers of louver angle 23° with flow depth of 16 and 24 rom. Louver pitch, louver length and fm height are 1.7 mm, 6.4 rom and 8.15 rom, respectively, and core size ofheat exchangers is 350x 255 rom. Test condition and method The heat exchanger is installed in the test section and insulation is placed around heat exchanger to protect it from heat loss and air leakage. The tests are performed in range of Reynolds number of 80-400 with water flow rate maintained at 320 kg/h. The inlet air dry and wet bulb temperatures of the heat exchangers are maintained at 27°C and 19°C, respectively, and the inlet water temperature is 6°C.