Frost formation on a vertical plate in simultaneously developing flow

It is well recognized that frost formation on heat exchanger surfaces seriously affects the performance of a refrigeration system. Consequently, defrosting is essential, yet it is only effective when both analytical tools and comprehensive experimental data on frost formation are available. In air conditioning units, frost formation most commonly takes place in the entrance region of the heat exchanger. Therefore, in this study, an experimental investigation was undertaken to characterize the effect of environmental conditions on the frost growth occurring on a vertical plate in the hydrodynamically and thermally developing region. Several experiments were performed while four environmental parameters, inlet air temperature, inlet air humidity, air velocity and cooling surface temperature, were varied. The thickness, mass, and density of the frost layer were determined from the measured data and empirical correlations were reduced from dimensionless parameters.     

Experimental study on frost release on fin-and-tube heat exchangers by use of a novel anti-frosting paint

Frost formation on heat exchangers is an undesirable phenomenon that almost inevitably exists in refrigeration and cryogenic equipment; it can significantly affect the thermal efficiency of heat exchangers and reduce the performance of the refrigeration system. In this paper, a newly developed anti-frosting paint was used to spray on the heat exchanger fins with coating thickness of 30 μm, and a series of comparative experiments were conducted to test its effectiveness in restraining frost deposition under different repeated frosting–defrosting cycles. The experimental results demonstrated that the anti-frosting time of the coated heat exchanger was substantially longer when compared with the uncoated heat exchanger. In addition, there was no appreciable frost deposition on the coated fins surface during the whole test.     

Frost formation mechanism analysis and frost growth prediction on ground aircraft

Frost predictions are needed to help the deicing operation decide. The mechanism of frost formation on aircraft surface under icing conditions has been analyzed. A simple theoretical frost growth prediction model by heat and mass transfer analysis has been presented. It produces a method to forecast the frost growth tendency. An experimental system for atmospheric frost reproduction is also presented. Effects of aircraft surface temperatures, air temperature on the frost growth is evaluated by this model.     

A CFD model of frost formation based on dynamic meshes technique via secondary development of ANSYS fluent

To simulate the non-uniform frost growth in flow direction for humid air flowing through a freezing channel, a 2D numerical frosting model based on dynamic meshes technique is developed in the current work via the secondary development of commercial ANSYS Fluent. The computation domain consists of both frost layer and humid air regions, and the local heat and vapor fluxes at the surface of frost layer are determined by numerical temperature and vapor fraction fields in the humid air region rather than by empirical correlations. The frost layer is treated as a growing packed bed with heat and mass transfer dominated by molecular diffusion, where local absorption coefficient of vapor desublimation and local vapor fraction are both determined by solving the pseudo steady vapor diffusion equation with a source term theoretically. The interface of frost layer and humid air regions is treated as two walls for the iteration of its temperature, of which the humid air side is specified with the temperature equal to the frost-side counterpart and the frost side takes the heat flux including the extra latent heat caused by vapor deposit. User-defined functions are compiled to implement the above treatments to ANSYS Fluent. Frosting experiments in the literature are simulated with the current model for validation. How the profile of frost layer evolves with time in the frosting process is explored. The contours and profiles of velocity, temperature and vapor fraction are presented to discuss the effects of heat and mass transfer on frost formation. Numerical results demonstrate that the proposed CFD model can predict the frost growth and densification with a relative deviation less than 5% compared with experiments. Besides, the computation load of current model is small due to no solution of complex multiphase flow. In addition, dynamic meshes help current model to capture the interface of frost layer and humid air regions accurately.     

Frost formation in vertical channels under natural convection

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 paper, frost growth on a cold, vertical plate in free convection has been experimentally investigated. The cold plate (0.095 m high, 0.282 m wide) was placed in vertical channels open at the top and bottom in order to permit the natural circulation of ambient air. The channels, rectangular in shape, were 2.395 m high and 0.36 m wide, with the depth set equal either to 20 mm, or 10 mm, or 6 mm in order to infer the influence of channel flow area on the natural convection and frost formation. 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). Several quantities (thickness, temperature and mass of frost, heat flux at the cold plate), were measured during the time-evolution of the process (7.5 h from the frost growth inception), and 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 frost mass, thickness and density data.     

Theoretical investigation on thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger

Based on the heat transfer characteristics of absorber plate and the heat transfer effectiveness-number of heat transfer unit method of heat exchanger, a new theoretical method of analyzing the thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger has been put forward and validated by comparisons with the experimental and numerical results in pre-existing literature. The proposed theoretical method can be used to analyze and discuss the influence of relevant parameters on the thermal performance of heat pipe flat plate solar collector.     

Mesoscale investigation of frost formation on a cold surface

Frost formation on a horizontal flat copper surface was experimentally investigated using microscopic observations. The experiments were carried out on −20 to 0 °C copper surfaces with 22 °C air and 15–85% relative humidities. The experiments showed that the frost formation on a cold surface generally begins with the formation and growth of condensate droplets, freezing of the super-cooled condensate droplets, formation and growth of initial frost crystals on the frozen droplets, growth of frost crystals accompanied by the collapse of some of the crystals, and finally frost layer growth. The freezing onset time and diameter of the super-cooled condensate droplets were characterized. The initial frost crystals can be classified into four groups according to their appearance and shape, with the variations of the frost crystal shape as a function of the cold surface temperature and air humidity.     

Thermodynamic performance of a hybrid air cycle refrigeration system using a desiccant rotor

Due to the concern on global warming, the demand for a system using natural refrigerant is increasing and many researches have been devoted to develop systems with natural refrigerants. Among natural refrigerant systems, an air cycle system has emerged as one of alternatives of Freon gas system due to environmentally friendly feature in spite of the inherent low efficiency. To overcome the technical barrier, this study proposed combination of multiple systems as a hybrid cycle to achieve higher efficiency of an air cycle system. The hybrid air cycle adopts a humidity control units such as an adsorber and a desorber to obtain the cooling effect from latent heat as well as sensible heat. To investigate the efficacy of the hybrid air cycle, the cooling performance of a hybrid air cycle is investigated analytically and experimentally. From the simulation result, it is found that COP of the hybrid air cycle is two times higher than that of the conventional air cycle. The experiments are conducted on the performance of the desiccant system according to the rotation speed in the system and displayed the feasibility of the key element in the hybrid air cycle system. From the results, it is shown that the system efficiency can be enhanced by utilization of the exhausted heat through the ambient heat exchanger with advantage of controlling the humidity by the desiccant rotor.     

A study of frost growth and densification on flat surfaces

The present study advances a theoretical and experimental investigation of the frost growth and densification on flat surfaces. This study focuses on the most important factors affecting the frost formation process, i.e. the surrounding air temperature, humidity and velocity, and the surface temperature. The processes of frost growth and densification were investigated experimentally in order to provide a physical basis for the development of a theoretical model to predict the variation of the frost layer thickness and mass with time. The mathematical model was based on mass and energy balances within the frost layer, assuming the frost as a porous medium and accounting for the supersaturation of the moist air on the frost surface. The governing equations for mass and heat diffusion were integrated analytically, giving rise to a semi-algebraic formulation which requires numerical integration of only one time dependent ordinary differential equation. When compared with experimental data, the model predictions of the frost thickness as a function of time agreed to within ±10% error bands. The experimentally-validated model was then used to predict the frost layer growth and densification with respect to the operation conditions such as plate surface temperature, air stream temperature, humidity and velocity.     

An inverse geometry problem in estimating frost growth on an evaporating tube

 When humid air comes into contact with a surface whose temperature is below the dew point of water vapor in air and also below the freezing point, frost deposition takes place over the surface. The phenomena of the frost growth are very complicated and therefore it is very difficult to model mathematically the behavior of frost growth and predict it. In the present study a transient inverse geometry heat conduction problem (shape identification problem) is solved using the conjugate gradient method (CGM) and boundary element method (BEM)-based inverse algorithm to estimate the unknown irregular frost thickness and shape. Results obtained by using the CGM to estimate the frost growth are justified based on the numerical experiments. It is concluded that the accurate frost shape can be estimated by the CGM except for the initial and final time. The reason and improvement of this singularity are addressed. Finally the effects of reducing the number of sensors and increasing the measurement errors on the inverse solutions are discussed. Received on 25 September 2000 / Published online: 29 November 2001     

Shell-and-double concentric-tube heat exchangers

This study concerns a new type of heat exchangers, which is that of shell-and-double concentric-tube heat exchangers. These heat exchangers can be used in many specific applications such as air conditioning, waste heat recovery, chemical processing, pharmaceutical industries, power production, transport, distillation, food processing, cryogenics, etc. The case studies include both design calculations and performance calculations. It is demonstrated that the relative diameter sizes of the two tubes with respect to each other are the most important parameters that influence the heat exchanger size.     

A study on the correlation between the thermal contact conductance and effective factors in fin–tube heat exchangers with 9.52 mm tube

The thermal contact resistance is a principal parameter interfering with heat transfer in a fin–tube heat exchanger. However, the thermal contact resistance in the interface between tubes and fins has not been clearly investigated. The objective of the present study is to examine the thermal contact conductance for various fin–tube heat exchangers with tube diameter of 9.52 mm and to find a correlation between the thermal contact conductance and effective factors such as expansion ratio, fin type, fin spacing and hydrophilic coating. In this study, experiments have been conducted only to measure heat transfer rate between hot and cold water. To minimize heat loss to the ambient air by the natural convection fin–tube heat exchangers have been placed in an insulated vacuum chamber. Also, a numerical scheme has been employed to calculate the thermal contact conductance with the experimental data. As a result, a new correlation including the influences of expansion ratio, slit of fin and fin coating has been introduced, and the portion of each thermal resistance has been estimated in the fin–tube heat exchangers with 9.52 mm tube.     

用于低温风洞的新颖制冷方法

描述了用于低温风洞的新颖制冷系统,利用热交换器回收排气冷量预冷压缩空气,然后再用热分离器将其降至深低温作风洞气源．原理性实验结果证实新制冷方法的可行性．讨论了新制冷方法产生的有一定压力的低温空气作引射气源,引射驱动回流型风洞的特性．其制冷方法与现有低温风洞喷雾液氮制冷相比,由于仅需压缩空气而无需液氮,造价更便宜．更由于能量利用合理,效率高,因而运行成本可显著降低．     

Internal flow numerical simulation of a cross‐flow fan in refrigerant operating condition using user‐defined functions

In the paper, a cross‐flow fan in refrigerant operating condition is systematically simulated using user‐defined functions. Three‐dimensional simulations are acquired with Navier–Stokes equations coupled with k–ε turbulence model, and internal flow characteristics of an indoor split‐type air conditioner are obtained, which is mainly composed of cross‐flow fan and heat exchanger. It has systematically been simulated in the isothermal flow condition that the performance of cross‐flow fan may be reduced easily with dry or humid air, and in the refrigerant operating condition in which user‐defined functions are applied to the humid air, considered as a mixture of dry air and vapor. A density‐modulated function is adopted to deal with the condensation of the vapor at the heat‐transfer region approximately. The results show flow mechanism of the two gas‐phase flow, including phase‐vary process. The distribution of the parameters is not uniform at the inlet of the machine, the intensity and position of pressure and velocity vary along the axial direction of the fan, the distribution of vapor volume fraction and turbulent intensity in heat‐transfer region is obtained, and the external characteristic data of the indoor machine are obtained and analyzed. Compared with the experimental data, the calculated characteristic curves and designed parameters are on target. © British Crown Copyright 2010/MOD. Reproduced with permission. Published by John Wiley & Sons, Ltd.     

Thermal behavior of spiral fin-and-tube heat exchanger having fly ash deposit

This research investigates the effect of fly-ash deposit on thermal performance of a cross-flow heat exchanger having a set of spiral finned-tubes as a heat transfer surface. A stream of warm air having high content of fly-ash is exchanging heat with a cool water stream in the tubes. In this study, the temperature of the heat exchanger surface is lower than the dew point temperature of air, thus there is condensation of moisture in the air stream on the heat exchanger surface. The affecting parameters such as the fin spacing, the air mass flow rate, the fly-ash mass flow rate and the inlet temperature of warm air are varied while the volume flow rate and the inlet temperature of the cold water stream are kept constant at 10 l/min and 5 °C, respectively.

From the experiment, it is found that as the testing period is shorter than 8 h the thermal resistance due to the fouling increases with time. Moreover, the deposit of fly-ash on the heat transfer surface is directly proportional to the dust–air ratio and the amount of condensate on heat exchange surface. However, the deposit of fly-ash is inversely proportional to the fin spacing. The empirical model for evaluating the thermal resistance is also developed in this work and the simulated results agree well with those of the measured data.     

Heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a heat exchanger

This paper describes heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a fin-tube heat exchanger. The experiments in which the moist air was passed into the heat exchanger coated with sorption material were conducted under various conditions of air flow rate (0.5–1.0 m/s) and the temperature of brine (14–20°C) that was the heat transfer fluid to cool the air flow in the dehumidifying process. It is found that the sorption rate of vapor is affected by the air flow rate and the brine temperature. Meanwhile, the attempt of clarifying the sorption mechanism is also conducted. Finally the average mass transfer coefficient of the organic sorbent coated on heat transfer surface of a fin-tube heat exchanger is non-dimensionalzed as a function of Reynolds number and non-dimensional temperature, and it is found that the effect of non-dimensional temperature on them is larger than Reynolds number .     

An engineering procedure for air side performance evaluation of flat tube heat exchangers with louvered fins

An accurate evaluation of possible air side heat transfer surface geometries is a prerequisite for optimal heat exchanger design. Aiming for practical engineering applicability a simplified and transparent analytical procedure for the assessment of louvered fin and flat tube heat exchanger geometries and the calculation of fin parameters that enable maximal performance for given boundary conditions has been developed. The proposed method comprises determining fins temperature profiles and effective heat transfer temperature difference, introduction of a relative heat transfer surface area, as well as the utilization of recent experimentally obtained heat transfer correlations confirmed for the observed range of boundary conditions. The proposed methodology is validated through comparison with experimental and numerical results of other authors.     

High temperature heat exchanger studies for applications to gas turbines

Growing demand for environmentally friendly aero gas-turbine engines with lower emissions and improved specific fuel consumption can be met by incorporating heat exchangers into gas turbines. Relevant researches in such areas as the design of a heat exchanger matrix, materials selection, manufacturing technology, and optimization by a variety of researchers have been reviewed in this paper. Based on results reported in previous studies, potential heat exchanger designs for an aero gas turbine recuperator, intercooler, and cooling-air cooler are suggested.     

Experimental investigation of plastic finned-tube heat exchangers,with emphasis on material thermal conductivity

In this paper, two modified types of polypropylene (PP) with high thermal conductivity up to 2.3 W/m K and 16.5 W/m K are used to manufacture the finned-tube heat exchangers, which are prospected to be used in liquid desiccant air conditioning, heat recovery, water source heat pump, sea water desalination, etc. A third plastic heat exchanger is also manufactured with ordinary PP for validation and comparison. Experiments are carried out to determine the thermal performance of the plastic heat exchangers. It is found that the plastic finned-tube heat exchanger with thermal conductivity of 16.5 W/m K can achieve overall heat transfer coefficient of 34 W/m² K. The experimental results are compared with calculation and they agree well with each other. Finally, the effect of material thermal conductivity on heat exchanger thermal performance is studied in detail. The results show that there is a threshold value of material thermal conductivity. Below this value improving thermal conductivity can considerably improve the heat exchanger performance while over this value improving thermal conductivity contributes very little to performance enhancement. For the finned-tube heat exchanger designed in this paper, when the plastic thermal conductivity can reach over 15 W/m K, it can achieve more than 95% of the titanium heat exchanger performance and 84% of the aluminum or copper heat exchanger performance with the same dimension.     

A method for measuring the creep behavior of pressurized polymer tubing

Polymer components have been proposed for use in domestic solar hot-water heating systems. A polymer heat exchanger is under development for such systems. For heat transfer considerations, the heat exchanger will comprise many thin-walled tubes. The heat exchanger must survive 10 years of service at high pressure (1.55 MPa) and high temperature (82°C). A novel method has been developed for evaluating the long-term performance (creep) of the polymer tubing. Traditional creep testing, performed with dog bone test specimens, cannot be applied because the thin-walled tubing has anisotropic material properties. Consequently, performance must be evaluated directly on the extruded tubing. The method entails wrapping a Constantan wire around the tube specimen to continuously record the hoop strain. For pressure loading of tubing, this method offers significant improvements over strain gage instrumentation. In this paper, the test method is described, an analysis of the strain transfer between the tubing and wire wrap is presented, and strain data for polypropylene tubing measured with a strain gage and wire wrap are compared. The data show that the wire measurement method can be successfully used for the characterization of long-term mechanical behavior of polymer tubes.