Frost Weight Research Articles

Frost formation non-uniformity in a multi-circuit V-shape evaporator

Formation of frost on outdoor evaporators poses a significant challenge for air source heat pumps operating in cold climates. It is widely recognized that uneven air-flow across an evaporator can have a negative impact on both the evaporator’s performance and the overall efficiency of the heat pump. This study investigates the influence of non-uniform air-flow distribution on a V-shaped outdoor evaporator as a component of an air-source transcritical CO2 heat pump when subjected to frosting conditions. To this end, we developed and validated against independent experiments a numerical frost-formation model, and used it to study a multi-circuit evaporator of interest under different ambient conditions. It is shown that the uneven air-flow distribution results in varying frost formation patterns across different circuits of the heat exchanger, including differences in the amount of frost accumulated and its distribution among the individual tubes; the circuit with the largest frost weight accumulates 18% more frost compared to the one with the smallest in one hour under the sever frosting condition. Non-uniform frost formation also leads to air-flow distribution changing over time, and the rate at which this change occurs depends on the ambient conditions. For the presently investigated case, frost formation suppresses the non-uniformity of air-flow distribution. Moreover, evaporators with non-uniform and ideal uniform air-flows are compared and it is revealed that the difference in their capacities is more noticeable at the beginning of the frosting period.

Frost suppression performance of an air source heat pump using sensible heat from indoor air to preheat outdoor air

Frost forms on the surfaces of air source heat pump (ASHP) outdoor heat exchangers under heating mode in winter, when the surface temperature of the heat exchanger is lower than both 0°C and the ambient dew point temperature. This can reduce the coefficient of performance (COP) and heating capacity of the ASHP unit. A novel ASHP was examined using two rooms with the same size, orientation, and structure. To test the frost suppression performance of the novel system, two ASHPs of the same size and specifications, but one including a sensible-heat preheating device, were placed in the test platform rooms. By comparing and analyzing field-measured parameters such as COP, heat loss, and frost weight of the two ASHP units, the frost suppression performance of the novel ASHP was evaluated in severe, moderate, and mild frost areas. The results show that when the air conditions were −3.9°C and 98.7% in the moderate frost area and 3.2°C and 96.7% in the severe frost area, the novel ASHP showed significantly improved performance. The frost formation time increased from 25 min to 38.5 min, frost-defrost heat loss decreased by 31.6%–43.7%, frost weight decreased 11.8%–17.4%, and COP increased 7.4%–13.1% compared with those of the conventional ASHP. In the low temperature and mild frost area, when the air temperature was −11.8°C and the relative humidity was 88.8%, the frost suppression effect of the novel ASHP was unapparent. Affected by the climate, the operation performance of ASHP will be different. Different ASHP can be used to adapt to the weather in different regions. The novel ASHP can significantly suppress frost and improve the heating performance of the unit in the mild climate and frost serious areas.

Experimental Investigation on Evaporator Surface Modification for Hydrophobicity and Frost Resistance

Frost accumulated on the surface of the evaporator leads to the increase of heat transfer resistance between the refrigerant and environmental medium, which reduces the system performance of cold storage and air source heat pump. To solve this problem, the surface-modified evaporator can achieve retardation of the frost formation, which greatly improves the heat exchange effect between the refrigerant and the external environment. In this paper, a novel coating sprayed on the evaporator was developed using the sol-gel method and the hydrophobicity, durability, and thermal conductivity were investigated. Moreover, a modified method of frost weight measurement under different temperatures and humidity was first proposed. The results showed that the coating has a favorable hydrophobic effect and thermal conductivity, and long service life. Under the standard experimental condition, the frost weight on the coating surface can be reduced up to 26.1 %. Additionally, the effect of coating composition ratio on its performance was further investigated. To sum up, this new coating has the potential for industrialized application.

Mechanisms of trace water vapor frosting on a cryogenic surface in nitrogen gas flow

Water vapor contamination control is currently one of the key technical issues faced in cryogenic wind tunnels. However, the behaviors and the underlying mechanisms of trace water vapor frosting under cryogenic conditions still remain unknown due to the extreme experimental conditions and difficulties in direct measurement. A computational fluid dynamics (CFD) model was developed for describing the frosting process of trace water vapor on a cryogenic surface in nitrogen gas flow. The calculated results were validated against published experimental data and showed good agreements. The macroscopic and local growth behaviors of the frost layer were further studied based on the validated model. The variation in the nitrogen gas density under cryogenic conditions and its impact on the frost layer properties were analyzed. The change in the nitrogen gas density displayed a major difference between the frosting under refrigeration temperature conditions and cryogenic conditions, and taking the nitrogen gas density as a constant like previous models would cause great deviations in the calculated frost layer properties. The influence of water vapor content on the frost properties was also investigated. The frost layer thickness showed less sensitivity to changes in the water vapor content compared to the frost weight deposited on per unit area of the cold surface and the frost layer density. The frost layer under extremely low water vapor content might still cause small levels of distributed roughness on the surfaces, thereby affecting the aerodynamic characteristics of the model in the cryogenic wind tunnel.

The Inhibition of Icing and Frosting on Glass Surfaces by the Coating of Polyethylene Glycol and Polypeptide Mimicking Antifreeze Protein.

The development of anti-icing, anti-frosting transparent plates is important for many reasons, such as poor visibility through the ice-covered windshields of vehicles. We have fabricated new glass surfaces coated with polypeptides which mimic a part of winter flounder antifreeze protein. We adopted glutaraldehyde and polyethylene glycol as linkers between these polypeptides and silane coupling agents applied to the glass surfaces. We have measured the contact angle, the temperature of water droplets on the cooling surfaces, and the frost weight. In addition, we have conducted surface roughness observation and surface elemental analysis. It was found that peaks in the height profile, obtained with the atomic force microscope for the polypeptide-coated surface with polyethylene glycol, were much higher than those for the surface without the polypeptide. This shows the adhesion of many polypeptide aggregates to the polyethylene glycol locally. The average supercooling temperature of the droplet for the polypeptide-coated surface with the polyethylene glycol was lower than for the polypeptide-coated surface with glutaraldehyde and the polyethylene-glycol-coated surface without the polypeptide. In addition, the average weight of frost cover on the specimen was lowest for the polypeptide-coated surface with the polyethylene glycol. These results argue for the effects of combined polyethylene glycol and polypeptide aggregates on the locations of ice nuclei and condensation droplets. Thus, this polypeptide-coating with the polyethylene glycol is a potential contender to improve the anti-icing and anti-frosting of glasses.

Experimental and numerical investigations of frost formation on wavy plates

In this work, frost formation on wavy plates with −5 to 2 °C air temperatures, 85% air relative humidity, 0.31–0.92 m/s air velocities and −20 to −5 °C cooling surface temperatures was experimentally and numerically investigated. In the experiments, the frost layer growth process was recorded with frost thickness and weight measured. The experimental results show that the frost thickness increases with time and the frost layer grows faster with higher phase change driving force and higher water vapor mass flux. In the simulations, a frosting model was implemented as the source term in the Euler multi-phase flow method to simulate frosting process. The simulation results of frost layer growth show that densest frost appears at the leading edge of the plate. The frost density on the peaks of the plate is larger than that on the bottom of the valley. The simulations show that the air velocity increases with the frost layer growing. The simulated frost profile, frost weight and frost thickness agree well with the experimental results, which means the frosting model can be used to predict frost formation on wavy plates.

Frosting model based on phase change driving force

Based on the Gibbs free energy, a non-dimensional phase change driving force for frosting was proposed. A frosting model with the non-dimensional phase change driving force and a criterion for calculating the frost growth was developed. By combining the frosting model into the Euler multi-phase flow method, the local frost formation and the humid air flow characteristics can be simulated. The frosting model was validated by comparing not only the simulated frost thickness but also the corresponding frost weight with experimental data for various conditions. Moreover, the model was also validated by comparing the localized temperature variation with experimental data. The frosting model predicts the frost density distribution, temperature distribution, humid air velocity distribution and the mass transfer rate. The results show that the frost thickness and frost weight grow with time. In the frontal region, the water vapor mass fraction is larger and the mass transfer rate is larger, which leads to denser frost. The different characteristics of heat transfer in the frost layer region and the humid air region can be reflected by the simulated temperature distribution. The simulated velocity distribution shows that the air velocity is close to zero in the frost layer and the velocity in humid air region increases as the frost grows. The simulated localized temperature, localized frost thickness, frost profile, averaged frost thickness and frost weight have a good agreement with experimental results.

Phase change mass transfer model for frost growth and densification

A frosting mechanism was used to develop a phase change mass transfer model to predict the frost layer growth and densification. The model with a criterion can describe the mass transfer of water vapor from the humid air to increase the frost thickness and the frost density. Frost formation on a cold surface with local cooling was simulated by using the model as a source term in FLUENT to predict the frost morphology, temperature distribution and frost weight which are all in good agreement with the experimental results. The results show that the frost first forms around the cooling block and then extends gradually to other directions, with the frost above the cooling block having the greatest density. The average frost thickness increases gradually with time with the growth rate slowing down, while the average frost density increases with the increases rate getting faster. The temperature distribution in the computational domain is related to the frost layer morphology and the frost surface temperature is usually lower than the freezing point temperature. As the frost layer grows, the average air velocity increases with the maximum local velocity located above the thickest part of the frost layer.

Experimental study of frost formation on cold surfaces with various fin layouts

Frost formation significantly increases the heat transfer resistance on heat exchangers and the flow resistance in the channels, so there is great interest in understanding the frost formation mechanism on a finned surface. Frost formation was experimentally investigated on several complex finned surfaces, including a single aligned fin, a single angled fin, a single row of aligned fins, a single column of angled fins and five columns of angled fins. The experimental conditions were −2 to 7 °C air temperatures, 80% air relative humidity, 0.3 to 1.1 m/s air velocities and −19 to −10 °C cold surface temperatures. The results show for all the investigated pieces, the frost weight per unit area in the fin region is greater than in the base region at a lower temperature, more frost was observed near the edges than in the middle of the fins, the fins angled with the airflow had more frost than the aligned fins and the multiple lines and columns of fins had the most frost on the frontal fins. A dimensionless empirical correlation for the frost weight in the fin area was developed with 85% of predictions within ±30% of the experimental data.

Experimental investigation of adverse effect of frost formation on microchannel evaporators, part 1: Effect of fin geometry and environmental effects

This study experimentally investigated the frost growth on louvered folded fins in microchannel heat exchangers when used in outdoor air-source heat pump systems. The effects of surface temperature, fin geometries, and air environmental conditions were studied. The overall aim was to isolate and quantify the effect of geometry from surface temperature effects. Experimental data of frost weight, local frost thickness, air pressure drop across the coils, time of frost–defrost cycles and heat transfer rates were recorded. Data showed that the frosting time and the frost growth rates depended mainly on the local fin surface temperature. Lower fin density was beneficial because it delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. The air humidity had a fairly significant effect on rate of frost formation while air velocity seemed to have a small effect on the frost growth rate.

Experimental investigation of the surface temperature and water retention effects on the frosting performance of a compact microchannel heat exchanger for heat pump systems

Frost formation on a louvered fin microchannel heat exchanger was experimentally investigated in this paper with the aim of determining the dominant factors affecting the time of frosting and frost growth rate. A novel methodology was developed to measure frost thickness and frost weight at intervals during the frosting period. Frost mass and thickness growth rates, corresponding coil heat transfer, capacity degradations and air pressure drop are measured and discussed. The experimental data showed that at a given air dry bulb temperature, the fin surface temperature and air humidity are the primary parameters that influence the frost growth rates. Water retention and air velocity had a secondary impact on the frosting performance. From digital images of the frost growth it was observed that frost does not nucleate from the water droplets retained in between fins but it developed from the leading edges of the fins.

Prediction of the air temperature and humidity at the outlet of a cooling coil using neural networks

The main objective of this study is to predict air temperature and humidity at the outlet of a wire-on-tube type heat exchanger using neural networks. For this purpose, initially the heat exchanger was coupled to a refrigeration unit and placed in a wind tunnel. Afterwards, its performance was tested under various experimental conditions. We measured nine input parameters, namely, temperature and humidity of the air entering the coil, air velocity, frost weight, the temperature at the coil surface, mass flow rate of the heat transfer fluid and its temperatures at the inlet and outlet of the coil along with ambient temperature. Additionally, we measured temperature and humidity of the air leaving the coil as the output parameters. Then, a feed-forward neural network based on backpropagation algorithm was developed to model the thermal performance of the coil. The artificial neural network (ANN) was trained using the experimental data to predict the air conditions at the outlet of the coil. The predicted values are found to be in good agreement with the actual values from the experiments with mean relative errors less than 1% for outlet air temperature and 2% for outlet humidity. This demonstrates that the neural network presented can help the manufacturer predict the performance of cooling coils in air-conditioning systems under various operating conditions.

Frost Temperature Relations for Defrosting Sensing System

To develop a better defrosting control system on finned evaporators of a refrigeration system, a study is conducted to better quantify the frosting and defrosting processes by using an infrared thermometer to determine the frost surface temperature. For gradual frost deposition a slow variation in surface emissivity is expected, while the defrosting process is characterized by a sudden change of this property. As an indicator for the defrosting initiation control mechanism, the times at which the IR signals stabilize at different conditions (Reynolds number, cold surface temperature, and ambient temperature) are reported along with the terminal mass concentration of a defined frosting process. On the other hand, the abrupt variation of surface emissivity indicates the termination of the defrosting process. Removable fins are used to measure frost weight, and a video microscope is used to determine the frost thickness. Defrosting initiation time and durations marked by melting, temperature are reported as a function of initial mass concentration and defrosting base surface temperature. The presented results could be used to design a better defrosting control system with better accuracy and energy saving features.

Performance investigation of a finned tube evaporator under the oblique frontal air velocity distribution

The performance evaluation of the finned tube evaporator under different frosting boundary conditions is of great importance to the refrigeration industry. This paper presents a set of equations, which can be used to predict the performance parameters of an evaporator, when there is an oblique angle between the inlet air velocity and frontal face of the evaporator. In order to calculating the performance, a simulation model for predicting the performance of a plate-fin tube evaporator, on which frost formation occurs, has been presented. This model adopts different numerical algorithms according to different flow conditions including laminar, transitional and turbulent flow patterns. An experimental setup is built to verify the validity of this model. Then a comparison between the model's predictions and laboratory test data is provided. After correction, the numerical program based on this model is used to predict relationship between the oblique angel of the inlet air velocity and performance parameters (including frost weight, pressure drop and refrigerating capacity of the evaporator). At the end of this article, the degree of performance degradation is described by a set of equations that is obtained through regression analysis.