Effect Of Humidity And Inclination Angle On Microchannel Heat Exchanger Performance

The effect of inlet humidity conditions and inclination angle on the air side thermal hydraulic performance of a brazed aluminum heat exchanger has been investigated experimentally. The crossflow heat exchanger had flat tubes and folded fins with fin and louver pitch of 2.1 mm and 1.4 mm, respectively. The glycol temperature was held nearly isothermal at 1C, while humidity of the 12C inlet air varied from 0—80%. The Reynolds numbers based on louver pitch ranged from 80 to 400. This range of operating conditions permitted air-side heat transfer coefficients to be determined within ±15-20% for dry and wet conditions, respectively. Wet-surface heat transfer degradation was substantial and varied with Reynolds number, but was only a weak function of inclination angles less than 45 degrees. On the other hand, the wetted-surface pressure drop penalty was relatively small, only 314% across the range of inclination angles and Reynolds numbers. The results provide insights into the ability to maintain louver-directed flow under inclined and wet-surface donations. NOMENCLATURE A: area s: saturation state Cp: specific heat t: temperature Cr: capacity ratio δf: fin thickness Dh: hydraulic diameter θ: Inclination angle Fd: flow depth γ: aspect ratio h: heat transfer coefficient ε: effectiveness H: fin height η: Efficiency i: enthalpy Subscipts k: conductivity f: fin L: heat exchanger height i: glycol-side Lp: louver pitch o: air-side m& : mass flow rate w: wet-surface or water NTU: Number of transfer unit 1: input Q: heat transfer rate 2: output RH: relative humidity INTRODUCTION The effect of inlet humidity conditions and inclination angle on the air side thermal hydraulic performance of a brazed aluminum heat exchanger has been investigated experimentally. There are some publications on the effect of inclination angle and inlet humidity conditions on the heat transfer and pressure drop of the heat exchangers. However, most of the published data have considered bare-tube banks, high-fin tube banks and conventional finned tube heat exchangers (Groehn, 1983, Monherit et al., 1986, Moore et al., 1979, Aarde et al. 1993, Chang et al. 1994, Kedzierski, 1997, Mirth et al., 1993, Mirth et al., 1994, Wang et al. 1997, Wang et al., 2000). Mirth et al (1993, 1994) showed that inlet humidity conditions affected the heat exchanger performance. On the other hand, Wang et al. (1997, 2000) reported that they did not influence significantly the sensible heat transfer coefficients, while their effect on the pressure drops depended on the heat exchanger configurations, especially the longitudinal tube pitch. When the longitudinal tube pitch was 22 mm, the effect of inlet conditions was negligible, while for the longitudinal pitch of 19.05 mm the friction factors for RH =90% were 5-25% larger than those for RH=50%. A microchannel tube heat exchanger is one of the potential alternatives for replacing the conventional finned tube heat exchangers and has been considered as both evaporator and gas cooler for prototype CO2 airconditioning systems (Kim and Bullard, 2000). Many investigators have studied the air-side heat transfer and pressure drop characteristics of the louvered fin and flat tube heat exchangers (Sahnoun et al., 1992, Chang et al., 1996, Chang et al., 1997, Kim et al., 2000, Chiou et al., 1994, Kim and Bullard, 2002, McLaughlin et al., 2000). However, only small amount of published data on the effect of the inclination angle on the performance of the brazed aluminum heat exchangers is available in the open literature. Recently, Osada et al. (1999) studied the effect of inclination on the heat transfer and pressure drop characteristics of the louvered fin automotive evaporators with larger flow depth (Fd=58 and 70 mm) and conducted condensate visualization tests. They reported that both the leeward and windward inclinations improved heat exchanger performance. Kim et al. (2001) investigated the effect of inclination angle (0°, ±30°, ±45°, and ±60° clockwise) on the heat transfer and pressure drop of a brazed aluminum heat exchanger with Fd =20 mm under dry and wet conditions. They found that the heat transfer performance for both dry and wet conditions was not influenced significantly by the inclination angle (-60°<θ <60°), while the pressure drops increased consistently with the inclination angle. In our residential CO2 prototype system, the indoor heat exchanger was inclined to 67 off the vertical due to the space limitations in the wind tunnel (Beaver et al. 1999). The effect of inclination in those experiments was not known because the heat exchanger was too large to test at different angles. The purpose of this study is to provide experimental data on the effect of an inclination angle and inlet humidity conditions on the air-side thermal hydraulic performance for a smaller brazed aluminum heat exchanger under dry and wet surface conditions. A series of tests are conducted for the air-side Reynolds number range of 80-400 with variation of the inclination angles (0°, 14°, 30°, 45° and 67° clockwise) from the vertical position. The pressure drop characteristics are also addressed. A more detailed analysis of the wet surface data can be found in Kim et al. (2002), which also explores the issue of leeward and windward inclinations and louver directions at the inlet and outlet of the heat exchangers.