Frosting and defrosting characteristics of a fin according to surface contact angle

We investigated the characteristics of frosting and defrosting on a fin according to its surface contact angle under the winter operating conditions of a heat pump. The static contact angles were 2.5°, 75°, and 142° for hydrophilic, bare, and hydrophobic surfaces, respectively. The frost layer exhibited different frost formations for different surface contact angles during the early stages of frost formation. Frost retardation was observed on the hydrophobic fin, but the effect was not significant. The frost layer was thinner and the average frost density was higher on the hydrophilic surface than on the other surface-treated fins. The effect of surface treatment on defrosting time was found to be insignificant. However, the ratio of residual water on the hydrophilic surface was significantly smaller than those on the other surfaces. Because of this, in repeated frosting/defrosting experiments, the fin of the hydrophilic surface was found to show almost the same frosting/defrosting behaviors as those of the first run.     

Experimental study on the effect of frost parameters on domestic refrigerator finned tube evaporator coils

In this study, parameters affecting the frost formation on the evaporator of a refrigerator and the structure of frost were examined. Air velocity measurements both at the air inlet and outlet channels of the evaporator were performed, and the effect of air velocity on frost formation was examined. The rate of evaporation of water inside the refrigerator cabin was also recorded.     

Experimental investigations of frost release by hydrophilic surfaces

Frost formation occurs when water vapor in the surrounding air comes into contact with cold surfaces through heat and mass transfer. It is usually an undesirable phenomenon in most refrigeration and cryogenic systems. A few studies have shown that changing the surface energy, such as increasing the surface hydrophilicity or hydrophobicity, has significant effects on frost growth. In this paper, a kind of hydrophilic polymer paint is formulated to counteract frost deposition on cold surfaces. The coated surface can retard frost formation up to three hours under low plate temperatures (− 15.3°C) and high air humidity (72%). To test the antifrosting performance of the hydrophilic paint under more practical conditions, it is applied to a fin-and-tube heat exchanger and a domestic refrigerator at a coating thickness of 30 μm. Comparisons of frost deposition, pressure drops, and outlet temperatures are made between uncoated and coated heat exchangers. Under conditions of high air temperature (2.2°C) and relative high air humidity (90%), the paint prolongs the defrosting interval from 80 to 137 min. Experimental observations also show that the coated hydrophilic fins are free of frost deposition during the entire course of the test and that the coating has no significant additional thermal resistance.     

Performance characteristics of two combined ejector–absorption cycles

The paper describes the performance of an ammonia–water combined ejector–absorption cycle as refrigerator using two simple models. In the first an ejector draws vapour from an evaporator and discharges to a condenser. In the second, an ejector draws vapour from an evaporator and discharges to an absorber. The thermodynamics cycles and ejector operation on the temperature–entropy charts are shown. The thermodynamics of the combined ejector–absorption cycle are simulated by a suitable method and a corresponding computer code, based on analytic functions, describes the behaviour of the binary mixture NH₃–H₂O. It was found from the first model that the refrigerator (theoretical) coefficient of performance (COP) varied from 1.099 to 1.355 when the operation conditions were: generation temperature (237°C), condenser temperature (25.9–30.6°C), absorber temperature (48.6–59.1°C) and evaporator temperature (−1.1–7.7°C). In the second the theoretical COP vary from 0.274 to 0.382 when the operation conditions were: generation temperature (237°C), condenser temperature (91°C), absorber temperature (76.7–81°C) and evaporator temperature (−1.1–7.7°C).     

Advances in control of frost on evaporator coils with an applied electric field

This paper introduces an innovative technique on use of an applied electric field for control of frost over evaporator coils with fin density and geometric configuration of interest to freezer/refrigerator applications. The technique discussed in this paper, referred to as the “dielectric barrier discharge” (DBD) method, may be particularly suitable for application in evaporator coils with high fin density. Experiments conducted with a small-scale laboratory test-module, as well as a full-scale supermarket evaporator are presented. The DBD technique is based on generating localized non-resistive heating within fins of an evaporator coil via application of a high-voltage, alternating current through electrodes. Our experiments demonstrate that the defrosting time using DBD is substantially shorter than conventional techniques, while the energy consumption associated with the process is less than one half of the corresponding energy of the electrical resistance heating methods. Basic operational principles of the technique, its advantages and limitations when compared to conventional electrical defrosting techniques are discussed and presented for the first time in this paper.     

Temperature–enthalpy/entropy charts of combined ejector–absorption cycle using NH3–H2O

This paper describes the performance of an ammonia–water combine ejector–absorption cycle as refrigerator and heat pump. This combination brings together the advantages of absorption and ejector systems. Also, thermodynamic cycles on the temperature–enthalpy and temperature–entropy charts are shown. The thermodynamics of the combined ejector–absorption cycles are simulated by a suitable method and a corresponding computer code, based on analytic functions describing the behaviour of the binary mixture NH₃–H₂O. It is found that in the case of the refrigerator and heat pump, the theoretical coefficient of performance (COP) or the theoretical heat gain factor (HGF) vary from 1.6 to 90.4 per cent and 0.7 to 37.6 per cent, greater than those of the conventional absorption system, respectively. The operation conditions were: generator temperature (205.5 to 237.1°C), condenser temperature (25.9 to 37.4°C) and evaporator temperature (−8.4 to 5°C). Copyright © 2000 John Wiley & Sons, Ltd.     

Defrosting method adopting dual hot gas bypass for an air-to-air heat pump

A novel dual hot gas bypass defrosting (DHBD) method is developed to remove frost from the outside heat exchanger (HEX) of an air-to-air heat pump. The proposed method adopts two bypass lines of hot gas from the compressor: one is connected to the inlet of the outdoor HEX, and the other is connected to the outlet of the exchanger. We compare the dynamic performance and defrosting time of the conventional reverse cycle defrosting (RCD), hot gas bypass cycle defrosting (HGBD), and DHBD methods using a medium air-to-air 16 kW heat pump. The salient feature of the DHBD method is its ability to prevent a sharp decrease in the compressor outlet temperature at the melting frost stage after the HGBD process begins. Due to the additional bypass, the DHBD method sustained a higher compressor outlet pressure and reduced the defrosting time by 36% compared to the HGBD method. Compared to RCD, the defrosting time was comparable (126%); however, the amenity characteristics of the DHBD method were superior than those of the RCD method. The proposed DHBD method can overcome the main disadvantages of the RCD and HGBD methods, and showed excellent performance for an air-to-air heat pump in a defrosting operation.     

Frost formation on a bionic super‐hydrophobic surface under natural convection conditions

A bionic super‐hydrophobic surface has a multiple micro‐nano‐binary structure (MNBS) similar to the lotus leaf surface microstructure. This kind of surface has a contact angle of water greater than 150^° and a roll angle smaller than 5^°. In this paper, the frost deposition phenomena on a bionic super‐hydrophobic surface were observed. The surface has many micro bumps and its contact angle is 162^°. The formation of water droplets, the droplet freezing process, the formation of initial frost crystals and the frost layer structure on a cold bionic super‐hydrophobic surface under natural convection conditions were closely observed. The frost layer structure formed on the super‐hydrophobic surface shows remarkable differences to that on a plain copper surface: the structure is weaker, looser, thin, and easily removed and most importantly, it is of a very special pattern, a pattern similar to a chrysanthemum, a frost layer structure that has not been reported before to the best of the present authors knowledge. The experimental results also show that a super‐hydrophobic surface has a strong ability to restrain frost growth. The frost deposition on this bionic surface was delayed 55 minutes when compared with a plain copper surface under the conditions of a cold plate temperature of ?10.1^°C, air temperature of 18.4^°C, and relative humidity of 40%. A theoretical analysis was also presented to explain the observed phenomena. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 412–420, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20216     

Experimental investigation of frost formation on a parallel flow evaporator

This paper experimentally studied the frosting process of a folded–louvered-fin, parallel flow microchannel evaporator in a heat pump central air-conditioning system under three conditions, in which three open states of two capillaries were adopted. Surface temperature distribution on evaporator was measured by 16 thermocouples buried on the leeward side. Mesoscale frost formation processes on its front view surface for three different test conditions were observed using a Charge Coupled Device (CCD) camera. The results showed that the surface temperature distribution on the parallel flow evaporator was uneven and initial frost formation generally started on some partial surface areas of the louvered fins whose surface temperature was lowest after the heat pump system running 6 min later under conditions B and C, while the evaporator began to frost after 8 min under condition A. The non-uniform surface temperature distribution caused by the unequal distribution of the refrigerant flux in the flat tubes’ microchannels resulted in uneven distribution of frost. The ice crystals distribution and frost thickness in frost period could be obtained by the digital image processing method in which the initial pictures were converted into binary image. The results indicated that in a thin layer near fins’ surface, ice crystals had relatively high occupancy rate in the frost growing period and full growth period; the occupancy rate of ice crystals decreased almost linearly with the increasing of the frost thickness (frost height), where the decreasing rate in the frost full grown period was less than that in the frost growing period (in 0.1 mm frost thickness condition, the occupancy rate of ice crystals decreased to 58% in the frost growing period, while in the frost full grown period occupancy rate of ice crystals decreased to 90%; and in 0.25 mm frost thickness, they were 0%, 45% respectively). Furthermore, it can be found that the frost thickness increased as the time increasing and then finally reached maximum values of 0.3 mm, 0.35 mm, and 0.32 mm respectively at three conditions.     

Experimental investigation of frost issue on various evaporators having different fin types

Frost formation and growth is quite important physical phenomena in evaporators since frost can directly affect both capacity and fan performance. In this study, an in-house-code was programmed to determine frost ratio on evaporator surfaces via an image processing technique. Air inlet temperatures along with pressure drop across the evaporators were monitored and both temperature and pressure breaking points were identified for evaporators with flat, hydrophilic flat and corrugated fins. Evaporator capacities and pressure drops across the evaporators were experimentally measured and compared with theoretical findings. It was revealed that the evaporator with hydrophilic fin was the most suitable selection when entire measured and calculated parameters were considered altogether.     

Research on a combined adsorption heating and cooling system

A combined cycle capable of heating and adsorption refrigeration is proposed, and the experimental prototype has been installed. The system consists of a heater, a water bath, an activated carbon–methanol adsorption bed and a ice box. This system has been tested with electric heating, and has been found that with 61 MJ heating, the 120 kg water in the bath can be heated up from 22 to 92 °C meanwhile 9 kg ice of −1.5 °C is made. The calculated COP_system is 0.0591 and COP_cycle is 0.41. After reconstruction to a real hybrid household water heater–refrigerator, when 55 MJ heating is added to 120 kg 21 °C water, and the condensing temperature is controlled at about 30 °C, the result is the 4 kg water contained inside the methanol refrigerant evaporator was iced to −2 °C, the cooling capacity of the ice and the refrigerant in the evaporator will maintain the 100 l cold box for about three days below 5 °C. The experiments show the potentials of the application of the solar powered hybrid water heater and refrigerator. Theoretical simulation has been done, which is in good agreement with experimental results. This research shows that the hybrid solar water heating and ice making is reasonable, and the combined cycle of heating and cooling is meaningful for real applications of adsorption systems.     

Performance Evaluation of R134a/DMF-Based Vapor Absorption Refrigeration System

This article describes an experimental investigation to measure performances of a vapor absorption refrigeration system of 1 ton of refrigeration capacity employing tetrafluoro ethane (R134a)/dimethyl formamide (DMF). Plate heat exchangers are used as system components for evaporator, condenser, absorber, generator, and solution heat exchanger. The bubble absorption principle is employed in the absorber. Hot water is used as a heat source to supply heat to the generator. Effects of operating parameters such as generator, condenser, and evaporator temperatures on system performance are investigated. System performance was compared with theoretically simulated performance. It was found that circulation ratio is lower at high generator and evaporator temperatures, whereas it is higher at higher condenser temperatures. The coefficient of performance is higher at high generator and evaporator temperatures, whereas it is lower at higher condenser temperatures. Experimental results indicate that with addition of a rectifier as well as improvement of vapor separation in the generator storage tank, the R134a/DMF-based vapor absorption refrigeration system with plate heat exchangers could be very competitive for applications ranging from –10°C to 10°C, with heat source temperature in the range of 80°C to 90°C and with cooling water as coolant for the absorber and condenser in a temperature range of 20°C to 35°C.     

Prediction of an effective cooling output of the fin-and-tube heat exchanger under frosting conditions

In the paper, an analysis of heat and mass transfer during frost formation on a fin-and-tube heat exchanger has been presented. For calculation of an exchanged heat flux, a transient two-dimensional mathematical model of frost formation has been developed and numerically solved. The mathematical model and numerical procedure have been experimentally validated. For determination of an effectively exchanged heat flux inside the heat exchanger, the influence of the augmented heat transfer resistance and impact of the defrosting process have been taken into account. A detailed calculation of the frost growth rate has been a base for determination of heat transfer resistance of the frost layer. The influence of frost formation on the overall heat transfer coefficient has also been analysed. The effective exchanged heat flux has been calculated for different operating conditions, durations of cooling cycle as well as defrosting heat fluxes. Results have shown that the effectively exchanged heat flux significantly depends on operating conditions, such as air humidity and temperature, as well as the cooling cycle duration.     

Defrost improvement by heat pump refrigerant charge compensating

During winters, the air-source heat pump often operates with substantial frost formation on the outdoor heat exchanger, and the frost layer has to be melted away periodically to keep a high heat pump coefficient of performance (COP). Otherwise, the unmelted frost layer and water will become high density frost or ice layer in heating mode. However, it is difficult to melt the frost layer in the defrosting cycle, where the effective defrosting time plays an important role in improving the defrosting ability. Generally, the defrosting time can be decreased by the following ways: increasing the refrigerant flow rate effectively, and rapidly establishing the suction pressure, discharge pressure, and the compressor power. A new heat pump defrost system with a refrigerant charge compensator, instead of the accumulator which is a key component for the frosting cycle performance, is developed in this paper. Furthermore, test results showed that the improved frost system with the compensator worked as expected, and its suction and discharge pressures and the power of the compressor during the defrosting were much larger than before.     

Frost formation on a plate with different surface hydrophilicity

The objectives of this study are to develop frost maps for two different surfaces having two different hydrophilic characteristics and to find ambient conditions associated with the formation of frost structures. Test samples with two different surfaces having dynamic contact angle (DCA) of 23° and 88° were installed in a wind tunnel and exposed to a humid airflow. Frost structure is observed with a visualization system in the operating conditions of household refrigerator: airflow temperature in the range of +10-20 °C, humidity in the range of 2.64-9.36 g/kg^′, Reynolds number in the range of 7000-17,000 and cold plate temperature in the range of −11.6 to −28.4 °C. As results of this study, frost structures are classified and frost maps are proposed for two different surface hydrophilicities. Surface with low DCA (23°) shows lower frost thickness and higher frost density than that with high DCA (88°). It was found that frost structures on surfaces with different DCA are similar. However, low DCA surface at low humidity provides 20-30% denser frost formation due to the shift of areas with different structures.     

Numerical Simulation of the Performance of a Capillary Thermal Driven Ejector Refrigerator

A capillary driven ejector refrigerator is a new refrigeration system that can use solar energy and other low-grade heat sources. In this paper, the performance of the refrigeration system is simulated numerically by use of an iteration algorithm and block exchanging technology for all unit models. The flow and heat transfer characteristics in a solar collector, generator, ejector, condenser, and evaporator are analyzed and calculated. The results show that when the generating temperature is higher than 75–80°C and the environmental temperature is lower than 35°C, the system can work normally; the coefficient of performance of this refrigeration system is in the range of 0.05–0.15 by use of water as a refrigerant. The cooling capacity and COP increase with an increasing generative temperature and decreasing condensing pressure.     

Experimental study on the performance of air source heat pump system with multiple parallel outdoor units

This paper presents a novel air source heat pump for heating of buildings named air source heat pump with multiple parallel outdoor units (ASHPMO). Multiple outdoor units were connected in parallel with the aim of realising alternate defrosting and uninterrupted heating simultaneously. An experimental apparatus of the ASHPMO system was developed. The defrosting performance was experimentally investigated under different outdoor air temperatures, outdoor air relative humidity, and condensation temperatures, among other factors. The test results showed that the novel ASHPMO system could provide continuous heating when defrosting even under an outdoor air temperature of −10°C. Variations in compressor vapour suction and discharge pressure and temperature were observed. The minimum heating capacity could still reach 60% of that without defrosting. Under the defrosting condition with outdoor air temperature −10°C, both the heating coefficient of performance (heating COP) and total energy efficiency ratio (EER) of the system can reach to 2.0 and 2.32, respectively.     

Experimental and theoretical investigation of an innovative evaporative condenser for residential refrigerator

In this study, an innovative, evaporative condenser for residential refrigerator was introduced. A vapor compression cycle incorporating the proposed evaporative condenser was tested to evaluate the cycle performance. To allow for evaporative cooling, sheets of cloth were wrapped around condenser to suck the water from a water basin by capillary effect. The thermal properties at the different points of the refrigeration cycle were measured for typical operating conditions. The experimental results showed that the condenser temperature increases 0.45 °C for each degree increase in evaporator temperature when the air velocity is 2.5 m/s, and the ambient condition is 29 °C and the relative humidity is 37.5%. Meanwhile, the condenser temperature increase is 0.88 °C in the case of air velocity 1.1 m/s and ambient conditions of 31 °C and relative humidity of 47.1%. A theoretical model for the evaporative condenser was developed, and validated by experimental results. The theoretical model showed that the evaporative condenser can operate at a condensing temperature of 20 C lower than that of the air-cooled condenser for heat flux of 150 W/m², and at air velocity 3 m/s. The effect of the different parameters on the condenser temperature was studied too.     

Experimental and theoretical performance analysis of air-cooled plate-finned-tube evaporators

The determination of the cooling capacity and overall heat transfer coefficient of an evaporator is of great importance in refrigeration industry, so that, an investigation of the theoretical and experimental performance analysis of evaporators was carried out in this study. The experimental evaporator was analyzed in account with the most common and widely used correlations together with the parameters of air velocity, fin spacing, tube diameter, evaporator temperature, refrigerant type and frost height. After a good agreement between the experimental and theoretical results was obtained, the parameters which had not been able to investigated experimentally were analyzed theoretically. It is concluded that when the experimental and theoretical overall heat transfer coefficients were compared with those from the manufacturing catalogues (for the same working conditions), the latter was found to be 15–30% higher than the former one.     

Transient analysis and performance prediction of a solid adsorption solar refrigerator

The transient analysis and performance prediction of a solid adsorption solar refrigerator, using activated carbon/methanol adsorbent/adsorbate pair are presented. The mathematical model is based on the thermodynamics of the adsorption process, heat transfer in the collector plate/tube arrangement, and heat and mass transfers within the adsorbent/adsorbate pair. Its numerical model developed from finite element transformation of the resulting equations computes the collector plate and tube temperatures to within 5 °C. The condensate yield and coefficient of performance, COP, were predicted to within 5% and 9%, respectively. The resulting evaporator water temperature was also predicted to within 5 °C. Thus the model is considered a useful design tool for the refrigerator to avoid costly experimentation.