Minimising frost growth on cold surfaces exposed to humid air by means of crosslinked hydrophilic polymeric coatings

Experiments have shown that frost growth on cold surfaces exposed to warm humid air streams can be reduced significantly by means of crosslinked hydrophilic polymeric coatings. This derives from the ability of these materials, under frosting conditions, to absorb available water, and hence retard the growth of frost when compared to an uncoated metallic surface. The extent of reduction of frost growth appeared to vary with the water absorbing potential of the polymer-coat, as well as its water content prior to frosting. In general, measurements in over two hours of testing, indicated that the reduction in frost growth rate and subsequently frost thickness lies in the range, 10–30%. The absorbed water improves the thermal conductivity of the polymeric coating. This, consequently, lowers the total thermal resistance between the air stream and the cold plate, and hence retards the frost surface temperature in its rise towards 0°C, an effect that would prolong the effective operation time of the thermal process before there is need for defrosting. Further observations made in this study tend to suggest that the nature of the frost formed with an air stream of low relative humidity (RH) differed from that resulting from a high RH air stream.     

Dimensionless correlations of frost properties on a cold cylinder surface

We propose dimensionless correlations for frost properties on a cold cylinder surface. Frosting experiments were performed while changing various frosting parameters such as the air temperature, cold cylinder surface temperature, air velocity, and absolute humidity. The experimental data showed that a uniform frost layer grew around the circumference of the cylinder at a high air velocity. Dimensionless correlations for the thickness, density, and surface temperature of the frost layer, and for the heat transfer coefficient were obtained as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature. The applicable ranges of these correlations are Reynolds number of 700–3000 (air velocities of 0.5–2.0 m/s), Fourier number of 56.8–295.7 (operating time of 0–100 min), absolute humidity of 0.00280–0.00568 kg/kg_a, air temperatures of 3–9 °C, and cold cylinder surface temperatures of ?32 to ?20 °C. The proposed correlations agreed with the experimental data within an error of 15%.     

Prediction of the frost formation on a cold flat surface

A mathematical model is presented to predict the behavior of frost formation by simultaneously considering the air flow and the frost layer. The present model is validated by comparing with several other analytical models and our experiments. It is found that most of the previous models cause considerable errors depending on the working conditions or the correlations used in predicting the frost thickness growth, whereas the model in this work estimates the thickness, density, and surface temperature of the frost layer more accurately within an error of 10% except the early stage of frosting in comparison with the experimental data. Numerical results are presented for the variations of heat and mass transfer during the frost formation and for the behavior of frost layer growth along the direction of air flow. Also, a correlation between the convective heat and mass transfer is obtained as Le⁽¹⁻ⁿ⁾=0.905±0.005 in this work.     

Analysis of heat transfer and frost layer formation on a cryogenic tank wall exposed to the humid atmospheric air

In this paper heat transfer characteristics and frost layer formation are investigated numerically on the surface of a cryogenic oxidizer tank for a liquid propulsion rocket, where a frost layer could be a significant factor in maintaining oxidizer temperature within a required range. Frost formation is modeled by considering mass diffusion of water vapor in the air into the frost layer and various heat transfer modes such as natural and forced convection, latent heat, solar radiation of short wavelength, and ambient radiation of long wavelength. Computational results are first compared with the available measurements and show favorable agreement on thickness and effective thermal conductivity of the frost layer. In the case of the cryogenic tank, a series of parametric studies is presented in order to examine the effects of important parameters such as temperature and wind speed of ambient air, air humidity, and tank wall temperature on the frost layer formation and the amount of heat transfer into the tank. It is found that the heat transfer by solar radiation is significant and also that heat transfer strongly depends on air humidity, ambient air temperature, and wind speed but not tank wall temperature.     

A novel prediction model of frost growth on cold surface based on support vector machine

This paper presents a novel prediction model of frost growth on cold surface based on the support vector machine. The dataset used to develop and validate the presented model is obtained from the public literature. The predicted results are found to be in good agreement with the experimental data, with mean relative error 1.82% for the total heat flux, 2.65% for the frost mass concentration, and 5.15% for the frost thickness. Then, a sensitivity analysis of the frost growth model is used to investigate the effects of the operating condition parameters that influence frost growth. Finally, the total heat flux prediction model is selected as an example to investigate the models’ roughness by adding white noise in the input vectors and output targets of the training set, respectively, and together. The results show that the presented model is very suited to the frost growth prediction with high accuracy and good robust against noise, and accordingly it may help the manufacturer design an effective and energy-saving defrosting control strategy.     

铝表面结霜现象的实验研究

实验研究了自然对流时水平铝表面上的结霜现象.实验中空气温度25 ℃、湿度50%,冷面温度-40～0 ℃.与先前的铜表面实验结果类似,在冷面温度高于-38 ℃时,结霜前铝面上首先出现水珠;冷面温度越低,过冷水珠存续的时间越短,冻结温度也越低;冷面温度较高时,水珠冻结时引起的壁温回升较明显;不同冷面温度对应的初始霜晶形状不同;结霜过程中霜层表面新形成的霜晶形状随时间变化.将冷面温度降至-38 ℃后再与湿空气接触时,发现直接出现霜晶而未经过结露阶段.     

A one-dimensional model for frost formation on a cold flat surface

An analytical model is developed for the formulation of a frost layer on a cold flat surface by considering the molecular diffusion of water, and heat generation due to the sublimation of water-vapor in the frost layer. Heat generation in the frost layer is expressed in terms of water-vapor density and the absorption coefficient. To examine the validity of the present model the results from the present model are compared with experimental data. The predictions agree with the existing experimental data.     

Free convection frost growth in a narrow vertical channel

Processes involving heat transfer from a humid air stream to a cold plate, with simultaneous deposition of frost, are of great importance in a variety of refrigeration equipment. In this work, frost growth on a vertical plate in free convection has been experimentally investigated. The cold plate (0.095 m high, 0.282 m wide) was placed in a narrow (2.395 m high, 0.01 m deep) vertical channel open at the top and bottom in order to permit the natural circulation of ambient air. The cold plate temperature and the air relative humidity were varied in the −40 to −4 °C and 31–85% range, respectively, with the air temperature held fixed at 27 °C (±1 °C). The main quantities (thickness, temperature and mass of frost, heat flux at the cold plate), measured during the time evolution of the process, are presented as functions of the input parameters (relative humidity and cold plate temperature); in particular, the role exerted by the plate confinement on the frost growth is discussed. Data are recast in order to identify compact parameters able to correlate with good accuracy frost thickness, mass and density data.     

A study of thermal performance with frosting under forced convection on a flat plate: Prediction of frost growth using nonhomogeneous property

The purpose of this study is the evaluation of thermal performance of a heat exchanger with frosting and decision of optimal defrosting cycle. Because the increase of flow resistance is the principal factor of a drop of heat transfer performance with frosting, thermal performance characteristics were examined. Based on those experimental results, we proposed a one‐dimensional nonhomogeneous frost growth model, and compared it with the experimental data in time and space. In a nonhomogeneous model with the frost property distribution taken into account, density distribution similar to the experimental result can be predicted. It is possible to make prediction closer to the experimental result compared with the conventional homogeneous model with respect to the temporal variation of frost height. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 674–689, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10122     

Evaluation of a jet plate solar air heater

To achieve higher heat transfer from the absorber plate to the flowing air stream with an intention to increase the amount of the collected energy, and hence, to improve the efficiency of an air-based solar collector, a unique jet impingement concept has been advanced for evaluation in the present study. To investigate the effects of various geometrical parameters such as the ‘hole’ or ‘nozzle’ diameter on the jet plate, their interspacings, the nozzle height, the distance between the absorber and the jet plate and the operational parameter such as the velocity of air impinging out of the holes/nozzles on to the back side of the absorber surface on the performance parameters of the jet impingement concept air heater, a detailed theoretical parametric analysis has been made on the design for different mass flow rates of air and different lengths of air channel. A parallel study has also been carried out on a conventional parallel plate air heater in order to compare its air temperature increment and performance efficiency with those of the jet plate air heater. The gain in air temperature increment and performance efficiency of the jet-concept air heater over that of the parallel plate air heater with duct depth 10 cm and length 2 m is 15.5°C to 2.5°C and 26.5% to 19%, respectively, for air flow rates in the range 50 to 250 kg/hm².     

A Study of Under-expanded Moist Air Jet Impinging on a Flat Plate

When a gas expands through a convergent nozzle in which the ratio of the ambient to the stagnation pressures is higher than that of the critical one, the issuing jet from the nozzle is under-expanded. If a flat plate is placed normal to the jet at a certain distance from the nozzle, a detached shock wave is formed at a region between the nozzle exit and the plate. In general, supersonic moist air jet technologies with non-equilibrium condensation are very often applied to industrial manufacturing processes. In spite of the importance in major characteristics of the supersonic moist air jets impinging to a solid body, its qualitative characteristics are not known satisfactorily. In the present study, the effect of the non-equilibrium condensation on the under-expanded air jet impinging on a vertical flat plate is investigated numerically in the case with non-equilibrium condensation, frequency of oscillation for the flow field becomes larger than that without the non-equilibrium condensation, and amplitudes of static pressure become small compared with those of dry air. Furthermore, the numerical results are compared with experimental ones.     

Natural convection heat transfer from a hot rectangular and a square corrugated plate to a cold flat plate

IntroductionReduction of heat loss from the absorber plate of asolar collector through the cover plates improvescollector efficiency. Therefore, the natllral convectionheat loss across air layers bounded by tWo parallel platesis of special interest to the designers of solar collectors.Most of the investigations on heat transfer in aconfined space have been cAned out with parallel platesin horizontal and inclined positions. Hollands, et al.[l]experimentally investigated the heat trallsferchara…     

Performance enhancement of a subcooled cold storage air conditioning system

This article experimentally investigates the enhancement of thermal performance for an air conditioning system utilizing a cold storage unit as a subcooler. The cold storage unit is composed of an energy storage tank, liquid-side heat exchanger, suction-side heat exchanger and energy storage material (ESM), water. When the cooling load is lower than the nominal cooling capacity of the system, the cold storage unit can store extra cold energy of the system to subcool the condenser outlet refrigerant. Hence, both the cooling capacity and coefficient of performance (COP) of the system will be increased. This experiment tests the two operation modes: subcooled mode with energy storage and non-subcooled mode without energy storage. The results show that for fixed cooling loads at 3.05 kW, 3.5 kW and 3.95 kW, the COP of the subcooled mode are 16.0%, 15.6% and 14.1% higher than those of the non-subcooled mode, respectively. In the varied cooling load experiments, the COP of the subcooled cold storage air conditioning system is 15.3% higher than the conventional system.     

Effects of intake air temperature on SI engine emissions during a cold start

This paper presents the concept of preheating the intake air to reduce cold-start emissions from gasoline engines. The effects of intake air temperature on emissions from a gasoline engine were studied by using an air heater based on spark ignition. A light-duty vehicle test of cold-start emissions was carried out at an ambient temperature of?7°C according to New European Driving Cycle for Euro 3 and Euro 4 exhaust emission legislations. The results showed that preheating the intake air could effectively reduce both hydrocarbon (HC) and carbon monoxide (CO) emissions and improve fuel economy during a cold start. During idling conditions, the key phase of the HC and CO emissions was the first 40 s. With the aid of the air heater, cold-start HC and CO emissions from the vehicle were lower than the limit values in the Euro 3 and Euro 4 regulations.     

A mathematical model for atmospheric ice accretion and water flow on a cold surface

A mathematical model is developed to describe ice accretion and water flow on a cold substrate of arbitrary shape. It is shown how the model may be applied to practical substrate shapes, such as flat surface, cylinder and aerofoil. A numerical scheme to solve the governing equations is then described. Results are presented for an aerofoil under conditions appropriate to in-flight icing and for a cylinder in conditions for atmospheric icing.     

Efficiency of solar plate air heaters

The efficiency of a solar air heating collector (SHC) has been investigated using a very simple nonstationary plate heating model. It is shown that the time to reach to the operating efficiency in the SHC is quite long. It is found that in the air heating temperature range 40–60 °C the stationary efficiencies of the SHC can be from 0.5 to 0.37.     

An interferometric study of a cold air jet with by-pass streams issuing into still ambient air

A two-dimensional air jet with identical by-pass streams (all at ～0°C) issuing vertically upwards, with exit velocities <20 m s^?1, into virtually still, ambient air (at ～ 19°C) has been studied. The temperature fields in and around the jets were mapped interferometrically for a distance of 30 cm downstream of the nozzle. Studies were also made of a single-stream two-dimensional jet of equivalent nozzle width.The centre-line temperature decay of the jet with by-pass streams was less rapid for jets with ratios of by-pass to central jet stream velocities greater than 2·12 than that for the single stream jet of equivalent nozzle width. Thus the use of a by-pass configuration system for the forced convective cooling of constrained personnel, such as aircraft pilots, would result in lower impingement temperatures, and hence require less energy to achieve a given degree of cooling.     

Impacts of plate slits on flame acceleration of premixed methane/air in a closed tube

The paper aims at revealing the interaction of various numbers of premixed methane/air jet flames in a closed duct. In the experiment, a high-speed video camera and pressure transducers are used to study the flame structure and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with Power-Law combustion model is employed to investigate the interaction between the moving flame and vortices induced by the thin plate. The results demonstrate that the flame propagation for all plate configurations can be divided into four typical stages, i.e. hemispherical flame, finger-shaped flame, jet flame and bidirectional propagation flame. For three plate configurations, the jet flames merge together under the effect of the vortices, and the more slits with the same blockage ratio (BR) do not mean the stronger deflagration. It is observed that the peaks of flame tip speed and pressure growth rate decrease with the increase of the number of slits. The sub-grid scale combustion model, Power-Law model, coupled with sub-grid scale viscosity model, dynamic Smagorinsky-Lilly eddy viscosity model can well reproduce the flame propagation. By analyzing the numeric flow structure, the flame propagation mechanism of premixed methane/air flame propagation in a tube with various slits can be explained in the view of pure hydrodynamics.