Ma Teng-fei　ZHANG Di-di　LＩＵ Yu-qing　TIAN Xiao-liang

【Abstract】The heat and mass transfer characteristics under frosting on surface of heat exchanger were experimentally investigated in different conditions of air temperature， relative humidity， and face velocity. The heat transfer and heat transfer coefficient decreased faster with the high relative humidity， low air temperature and initial face velocity. The air pressure drop rose faster with the high relative humidity and low air velocity.

【Key words】Heat and mass transfer； Mathematical modeling； Phase transition

中圖分類號(hào)： TK172 文獻(xiàn)標(biāo)識(shí)碼： A 文章編號(hào)： 2095-2457（2018）03-0147-004

0 Introduction

Air source heat pump systems are used for heating and cooling buildings all year around. They are energy efficient， compact and have low installation cost. The fin-tube heat exchanger has been widely used as condenser and evaporator in the heat pump system. When moist air flows across cold heat exchanger surfaces whose temperatures are lower than the freezing temperature， condensation and frost formation easy occur on the heat exchanger surfaces. The frost layer increases the heat transfer resistance between the heat exchanger and air， further， degrades the performance of the heat exchanger and even results in the shutdown of the heat pump. Therefore， exploring frosting behaviors of a fin-tube heat exchanger are focused on by researchers worldwide.

The frost accumulation on the surface of evaporator was a complex unsteady heat and mass transfer process. Various studies had been conducted experimentally and numerically on frost formation. An experimental investigation was undertaken to characterize the effect of inlet air temperature， inlet air humidity， air velocity and cooling surface temperature on the frost growth by Lee[1]. The experimental result showed with higher air temperatures， the frost layer increased in mass amount and density， while decreasing in thickness. However， Lee[2] put out the higher air temperatures lead to faster frost growth through experimentally investigating frost formation and growth in a spirally-coiled circular fin-tube heat exchanger. Seker et al.[3]got the same result that frost growth faster with air temperature increase. Yan[4]showed the effects of temperature and relative humidity of air， flow rate of air， refrigerant temperature， fin pitch， and row number on heat transfer performance. Ye[5]defined the value of the air-side heat transfer coefficient at the maximum mass transfer rate as the critical air-side heat transfer coefficient. The frost growth was significantly retarded when the heat exchanger was operated under conditions that avoided the critical air-side heat transfer coefficient.

In order to protect the heat pump from the harm of frost formation， the effects of different inlet air temperature， relative humidity and air velocity on evaporator cooling capacity， air pressure drop and total heat transfer coefficient during frosting were studied. The performance of the finned tube heat exchanger under frosting process was experimentally studied with inlet air temperature 0～10℃、relative humidity 80%～90% and face velocity 2～2.5m/s. The present study predicted and verified the total heat transfer rates， the airside pressure drop and the heat transfer coefficient by applying the a news model that considered the reduction of face velocity and the increased air flow rates through each section due to frost formation.

1 Experimental system

1.1 Experimental methods and instruments

The experimental system consisted of four parts， including the air duct system， refrigeration cycle system， data acquisition system and measurement system. The schematic diagram of the experimental system was shown in Fig. 1. There were two refrigeration cycle circuits in the experimental system， which was composed of inverter compressor， evaporator， thermal expansion valve and air-cooled condenser. One of refrigeration cycle circuits provided cold energy for the test heat exchanger. Another circuit cooperated with electric heater and humidifier to adjust airside inlet parameter. The refrigerant flow rate that was located between test evaporator and condensate was measured by a Coriolis-type mass flow meter with an accuracy of ±0.3% of reading. The inlet and outlet temperatures of the refrigerant， and the inlet and outlet （dry-bulb and wet-bulb） temperatures of air were measured by pre-calibrated RTDs （Pt-1000） which have an accuracy of 0.2℃. The pressure difference before and after the heat exchanger was measured by the differential pressure transducer with an accuracy of ±0.23% of reading. The air flow rate was controlled by a frequency conversion fan， and the airflow rate was measured by the pressure difference before and after the nozzle， which was arranged at the air volume collection device. The frost thickness on the fin surface was calculated by using the image processing method.

The data were recorded every five minutes with the acquisition system that transmitted the data to the personal computer for further operation. The specific parameters of the heat exchanger were shown in Table.1. The baseline testing conditions for these parameters were shown in Table 2.

Fig.1 Schematic diagram of experimental set-up

Table1 Parameters of heat exchanger

Table2 Operating conditions of experiments

1.2 Data reduction

Since frosting process included both sensible and latent heat transfer for the air side， the heat transfer rate of the air side could be calculated by

Qair=Qsen+Qlat=maircp，air（Tair，in-Tair，out）+mair（dair，in-dair，out）isv（1）

The heat transfer rate of the refrigerant side could be computed by

Qref=mref（href，in-href，out）（2）

The preliminary tests showed that the differences between Qa and Qr without frost formation are within 5%. Therefore， Qtotal was adopted for the results presented in this paper.

Qtotal=■（3）

2 Experimental results

2.1 Effect of air temperature

Fig.2 Heat transfer change over time

Fig.3 Air Pressure drop change over time

A higher air temperature resulted in a higher temperature difference between the air and the refrigerant. In addition， the humidity ratio was higher for a higher air temperature with the same relative humidity. Generally speaking， the heat transfer rate increased as air temperature increased. The surface of the heat exchanger became warmer for a higher air temperature. However， the air contained more moisture. A higher surface temperature was detrimental to the frost formation， but a higher moisture was favorable for the frost growth. From Fig.2 and Fig.3， it was interesting to note that there was an increase in the pressure drop when the air temperature was increased from 2 to 3℃. This indicated that the amount of frost increased as the air temperature increases. Thus， the effect of the moisture was more important than the effect of the surface temperature on frost accumulation. However， the pressure drop decreased as the air temperature was increased from 3 to 9℃. It meant that the amount of frost decreased as air temperature increased. Obviously， the effect of the air temperature was dominant.

2.2 Effect of relative humidity

The effects of the air relative humidity on the performance of the heat exchanger and the simulation results that was compared with the experimental result were presented in Fig.4 and Fig.5. Initially， the heat transfer rate was very close to one another for 80% and 90% relative humidities. Air with a higher relative humidity had a higher moisture content and led to more frost formation. As a consequence， the heat transfer rate dropped more quickly for higher relative humidities. Fig. 5 showed the effects of the relative humidity on the pressure drop. As the relative humidity increased， there was a higher pressure drop across the heat exchanger.

Fig.4 Heat transfer change over time

Fig.5 Air Pressure drop change over time

2.3 Effect of the air velocity

Fig.6 Heat transfer change over time

Fig.7 Air Pressure drop change over time

Table3

Fig.6 and Fig.7 showed the effects of air flow rate on heat transfer and pressure drop characteristics of heat exchanger and the computer simulation results that was compared with experimental results. It is noted from Fig.6 that a higher air flow rate led to a higher heat transfer rate as expected. In the separate experimental ran， results showed that the frost grew more from the top half of the heat exchanger as time progressed. The amount of frost formation increased as air flow rate decreased. This was because the surface of the heat exchanger becomes colder for a lower flow rate due to a lower heat transfer rate. The trend concerning the effect of air flow rate on frost formation was consistent with the experiments of Senshu[6]. However， it was contradictory to those of Rite and Crawford[7]. A decrease in air flow rateresulted in an increase in the frosting rate， thus the heat transfer rate degraded faster. As shown in Fig. 7， the experimental data indicated that an increased flow rate resulted in a higher pressure drop initially. This was similar to the trends of dry heat exchangers.

3 Conclusion

The performance of flat plate finned tube heat exchangers under frosting conditions was investigated experimentally. The following conclusions were made： The frost formation was greater for a lower air flow rate and high relative humidity， and the influence of air temperature was non-linear. The rate of pressure drop increased rapidly as the relative humidity increased and the air flow rate decreased.

【Reference】

[1]Lee YB， Ro ST. Frost formation on a vertical plate in simultaneously developing flow[J].Experimental Thermal and Fluid Science， 2002，26（8）： 939-945.

[2]Lee SH， Lee M， Yoon WJ， Kim Y. Frost growth characteristics of spirally-coiled circular fin-tube heat exchangers under frosting conditions[J]. International Journal of Heat and Mass Transfer， 2013，64（Supplement C）： 1-9.

[3]Seker D， Karatas H， Egrican PDN. Frost formation on fin- and- tube heat exchangers. Part II-Experimental investigation of frost formation on fin- and- tube heat exchangers[J]. International Journal of Refrigeration， 2004，27（4）： 375-377.

[4]Yan W-M， Li H-Y， Wu Y-J， Lin J-Y， Chang W-R. Performance of finned tube heat exchangers operating under frosting conditions[J]. International Journal of Heat and Mass Transfer， 2003，46（5）： 871-877.

[5]Ye H-Y， Park J-S， Lee K-S. Frost retardation on fin-tube heat exchangers using mass transfer characteristics with respect to air velocity[J]. International Journal of Heat and Mass Transfer， 2014，79： 689-693.

[6]Senshu T， Yasuda H， Oguni K， Ishibane K. Heat pump performance under frosting conditions： Part I - Heat and mass transfer on cross-finned tube heat exchangers under frosting conditions[M]. 1990： 324-329.

[7]Ali DA， Crawford RR， Crawford RR， Investigator P， Conditioning TA， Inc B， Cerl USA. Effect of Frost Formation on Evaporator Performance in Domestic Refrigerator Freezers[J]. 1992.