Modelling three conventional cooling processes of cooked meat by finite element method

The finite element method is used to analyse the two-dimensional transient heat transfer problem for cooling cooked meats with an ellipsoid shape under different environments offered by slow air, air blast and water immersion cooling units. The variations in physical properties of the cooked meat and in operating conditions during the cooling processes are included in the model. The models are solved by a user-friendly computer program developed by the authors. The numerical solutions are compared with the experimental results. The comparison shows that the maximum deviation between the predicted and experimental core temperatures for all the cooling processes is within 2.9°C.     

Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers

A coupled heat and mass transfer model is developed to extract the transfer coefficients for falling-films from the measurements on a tubular absorber. The mass transfer coefficients obtained from the coupled model and the log-mean-difference approach agree within about 10%. For the heat transfer coefficient, the values given by the two models can differ quite significantly. The cooling water temperature distribution predicted by the coupled model agrees well with measurements. The transfer coefficients obtained from experimental measurements using the various methods reported in the literature show wide variations.     

Heat transfer characteristics of cooked meats using different cooling methodsCaractéristiques du transfert de chaleur de viandes cuites réfrigérées par des méthodes différentes

Cooling rate and heat transfer characteristics of cooked meats using four different cooling systems of vacuum cooling, air blast cooling, water immersion cooling and slow air cooling were investigated. The experimental results show that only the vacuum cooling can achieve the requirement of cooling the cooked meats from about 74 to 10°C within 2.5 h. The vacuum cooling shows different heat transfer characteristics during the cooling process, as compared with other cooling methods. Vacuum cooling rate is controlled by the evaporation rate of water from the cooked meats, while the cooling rates of the other three cooling methods are governed by the thermal conductivity of the cooked meats. Therefore, it is impossible for air blast, water immersion and slow air cooling to obtain high cooling rates since these three methods are different only in the convective heat transfer from the surface of the cooked meat to the cooling medium.     

An experimental study on frosting of laminar air flow on a cold surface with local cooling

This paper presents the heat and mass transfer characteristics of the humid airflow in frosting conditions. A flat plate of aluminum with cooling modules at the central region was used for the simulation of flat surface part of the fin of the heat exchanger. The local surface temperature of the plate and the local thickness and total mass of the frost on the plate were measured to analyze the heat and mass transfer characteristics. In order to analyze the frosting characteristics, an analysis algorithm was developed, which can provide the local air temperature, the frost surface temperature, the sensible and the latent heat flux distributions at the test plate. Also, by integrating the local heat flux distribution, the average heat flux characteristics were analyzed. The present experiment and analysis found that the characteristic of the upstream airflow was very different from that of the downstream airflow.     

Air-side performance of a wavy-finned-tube direct expansion cooling and dehumidifying air coil

Airside heat and mass transfer and fluid flow characteristics of a wavy-finned-tube direct expansion air coil under cooling and dehumidifying condition have been experimentally investigated. Experiments were carried out to study the effects of operating conditions such as: air temperature, air relative humidity, air face velocity, and evaporator pressure on the airside performance (cooling capacity, dehumidification capacity, pressure drop, and heat transfer coefficient) of the coil. Charts for coil wet conditions, partially wet or totally wet, were conducted to identify the coil wet conditions in terms of the operating conditions. Two techniques, enthalpy potential method and equivalent dry-bulb temperature method, were used to analyze the data and to deduce correlations for Colburn factors for the different coil wet conditions. Comparison between the correlations predictions of the two techniques was presented.     

Description and experimental results of a semi-indirect ceramic evaporative cooler

Evaporative cooling is used in industrial and air conditioning processes to reduce temperature in different fluids. Direct evaporation systems can lead to environmental problems such as Legionnaire's disease, and indirect systems reduce system efficiency.This work presents the manufacture, test bed set up and trials carried out on a ceramic evaporative cooling system which acts as a semi-indirect cooler. Depending on air characteristics, it may act as a sensible or enthalpic exchanger. The water cooled in a cooling tower, using the return air coming from the conditioned room (22 °C and 50% comfort conditions) goes through the ceramic pipes, exchanging sensible and latent heat with a current of outdoor air.The use of this recovery system is mainly in climates with a high temperature and humidity such as tropical environments where the system yields a decrease in supply air humidity, using the cooling power of return air.The tests presented show the system behaviour for various supply air conditions.     

A computational fluid dynamic model of the heat and moisture transfer during beef chilling

The unsteady heat and mass transfer process during beef carcass chilling was modelled for a three-dimensional beef carcass geometry. A three-step method was used to simulate the simultaneous heat and mass transfer process in order to reduce the computational time. In the first step, a steady state simulation of the flow field was conducted. In the second step, the local heat and mass transfer coefficients were calculated. Finally, the third step consists of the simultaneous heat and mass transfer process simulation on the meat carcass only. A separate 1-D grid was used to calculate the moisture diffusion in the meat. The simulation of a 20-h chilling run takes 5 days on a 2.5 GHz Pentium 4 computer. The model allows calculating and predicting the heat load, temperatures, weight loss and water activity. Local variations in the heat and mass transfer coefficients, temperature and water activity were found around the beef carcass. The CFD model gives temperature predictions that agree with experimental data better than any previous model. The weight loss tends to be over-predicted probably due to neglecting the resistance caused by the fat cover.     

Evaluation of heat and mass transfer coefficients in a gauze-type structured packing air dehumidifier operating with liquid desiccant

In this paper, high performance packing, namely, structured packing that has good heat and mass transfer characteristics, is proposed for dehumidification of air using liquid desiccants and for regeneration of liquid desiccants. In order to design a structured packing tower for liquid desiccant — air contacting operations, heat and mass transfer coefficients for each phase are required. This paper is concerned with the interface transfer of heat and mass when air is brought into contact with the liquid desiccant solution. A theoretical study of evaluating heat and mass coefficients in an air-desiccant contact system employing three liquid desiccants, namely calcium chloride, lithium chloride, and a mixture of 50% calcium chloride and 50% lithium chloride (called cost effective liquid desiccant, CELD) is investigated. Moreover, air phase transfer coefficients are correlated with flow rates of air and liquid and the temperature of air, whereas liquid phase coefficients are correlated with rates of air and liquid flow, and the temperature and concentration of the liquid. The findings for the three liquid desiccants are compared and discussed.     

CFD simulation of coupled heat and mass transfer through porous foods during vacuum cooling process

A numerical simulation by using a computational fluid dynamics (CFD) code is carried out to predict heat and mass transfer during vacuum cooling of porous foods on the basis of mathematical models of unsteady heat and mass transfer. The simulations allow the simultaneous prediction of temperature distribution, weight loss and moisture content of the meats at low saturation pressure throughout the chilling process. The simulations are also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, thermal shrinkage, and anisotropy of the food. The model is verified by vacuum cooling of cooked meats with cylindrical shape within an experimental vacuum cooler. A data file for pressure history was created from the experimental pressure values, which were applied in the simulations as the boundary condition of the surface temperature.     

Prediction of air coil performance under partially wet and totally wet cooling conditions using equivalent dry-bulb temperature method

This paper mainly deals with the cooling and dehumidifying performance of air coils used in refrigeration and air-conditioning systems. A new method, equivalent dry-bulb temperature (EDT) method, is proposed for calculating the heat and mass transfer and for predicting the cooling modes (totally wet, partially wet, and totally dry) of moist air over the coil surface. A numeric model is further developed and validated with the experimental data of plain fin air coils. The deviation in both the cooling capacity and the vapor condensate estimated by the model is within the range of ±10%, and the prediction for the cooling modes of moist air over the coil surfaces is fairly exact.     

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures in horizontal tubes

A study on the prediction of heat transfer coefficient and pressure drop of refrigerant mixtures is reported. Heat transfer coefficients and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis assuming evaporation in horizontal tubes. Results indicate that nucleate boiling is suppressed at qualities greater than 20% for all mixtures, and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8–10% increase in heat transfer coefficients. Some mixtures with large volatility difference exhibit as much as 55% reduction compared to R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32%) and reduced mass flow rates (23%). Other mixtures with moderate volatility difference exhibit 20–30% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall heat transfer coefficient of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10% in transport properties causes a change of less than 6% in heat transfer prediction.     

空调用闭式冷却塔的运行参数对其冷却性能的影响

建立了闭式冷却塔热质交换的数学模型，利用该模型并结合计算机编程对闭式冷却塔的冷却性能进行了计算机仿真。系统地研究了喷淋水量、空气湿球温度、空气流量、管内水流量对闭式冷却塔冷却性能的影响趋势，通过分析得到了一些有价值的结论，为闭式冷却塔的研制及优化运行提供了参考依据。     

Steady state CFD modeling of airflow, heat transfer and moisture loss in a commercial potato cold store

Storage loss beyond permissible limit is one of the most important problems in Indian potato cold stores, which has been hindering further growth of this industry. The losses in the stored potatoes have a direct relation to the intricate coupled transport phenomena of heat, mass and momentum transfer therein. Therefore, airflow, heat transfer and moisture loss was investigated in a potato cold store of commercial scale under steady state condition using the computational fluid dynamics technique. The developed CFD model was a two-dimensional simplification of the cold store. Heat and mass transfer at the cooling coils were not modeled, instead temperature and relative humidity in the air space were specified based on measured values. The model was validated in a commercial scale cold store and was found to be capable of predicting the air velocity as well as product temperature with an average accuracy of 19.5% and 0.5 °C, respectively and also the simulated average total moisture loss was found to be only 0.61% water (w.b.) higher than the experimental one for a storage period of 6 months. The main deficiencies of the airflow pattern which resulted in wide variations in temperature and moisture loss within the stored commodity can be investigated. The model located the probable zones of hot and cold spots, excessive product dehydration and moisture condensation within the storage facility, which might lead to qualitative and quantitative deterioration in stored product. This modeling tool could very well be applied to incorporate necessary design improvements with a view to improve the airflow distribution and heat transfer in order to limit the storage losses within the permissible limit.     

Development of a slug flow absorber working with ammonia–water mixture: part II—data reduction model for local heat and mass transfer characterization

This study deals with a data reduction model for clarifying experimental results of a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions. The data reduction model to obtain the local heat and mass transfer coefficient on the liquid side is proposed by using the drift flux model to analyze the flow characteristics. The control volume method and heat and mass transfer analogy are employed to solve the combined heat and mass transfer problem. As a result, it is found that the local heat and mass transfer coefficient on the liquid side of the absorber is greatly influenced by the flow pattern. The heat and mass transfer coefficient at the frost flow region is higher than that at the slug flow region due to flow disturbance and random fluctuation. The solution flow rate and gas flow rate have influence on the local heat and mass transfer coefficient at the frost flow region. However, it is insignificant at the slug flow region.     

An absorption based miniature heat pump system for electronics cooling

The development of an absorption based miniature heat pump system is motivated by the need for removal of increasing rates of heat from high performance electronic chips such as microprocessors. The goal of the present study is to keep the chip temperature near ambient temperature, while removing 100 W of heat load. Water/LiBr pair is used as the working fluid. A novel dual micro-channel array evaporator is adopted, which reduces both the mass flux through each micro-channel, as well as the channel length, thus reducing the pressure drop. Micro-channel arrays for the desorber and condenser are placed in intimate communication with each other using a hydrophobic membrane. This acts as a common interface between the desorber and the condenser to separate the water vapor from LiBr solution. The escaped water vapor is immediately cooled and condensed at the condenser side. For direct air cooling of condenser and absorber, offset strip fin arrays are used. The performance of the components and the entire system is numerically evaluated and discussed.     

Frost formation and heat transfer on a cold surface in ice fog

In this paper a semi-empirical model describing heat and mass transfer on a cold surface in humid air under supersaturated frosting conditions is presented. The lack of psychrometric data in the supersaturated zone of the psychrometric chart has historically impeded the ability of researchers to accurately predict heat and mass transfer in supersaturated air. The work described in this paper has been partially made possible by developing a systematic procedure to compute the properties of supersaturated air, especially in the low temperature zone of the psychrometric chart. Development of such a capability will allow us to predict the amount of frost collected, the frost deposition and heat transfer rates, frost thickness and surface temperature, and other important parameters.     

A dynamic model of heat and mass transfer in a double-bed adsorption machine with internal heat recovery

This paper presents the results of a predictive two-dimensional mathematical model of an adsorption cooling machine consisting of a double consolidated adsorbent bed with internal heat recovery. The results of a base-case, taken as a reference, demonstrated that the COP of the double bed adsorption refrigeration cycle increases with respect to the single bed configuration. However, it was verified that, in order to maximize also the specific power of the machine, the adsorbent beds must have proper thermo-physical properties.Consequently, a sensitivity analysis was carried out, studying the influence of the main heat and mass transfer parameters on the performance of the machine. The results obtained allowed us to define the adsorbent bed design that maximizes its heat and mass transfer properties, as well as the most profitable heat recovery conditions.     

Air-side thermal hydraulic performance of multi-louvered fin aluminum heat exchangers

An experimental study on the air-side heat transfer and pressure drop characteristics for multi-louvered fin and flat tube heat exchangers has been performed. For 45 heat exchangers with different louver angles (15–29°), fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm), a series of tests were conducted for the air-side Reynolds numbers of 100–600, at a constant tube-side water flow rate of 0.32 m³/h. The inlet temperatures of the air and water for heat exchangers were 21 and 45°C, respectively. The air-side thermal performance data were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluid unmixed conditions. The heat transfer coefficient and pressure drop data for heat exchangers with different geometrical configurations were reported in terms of Colburn j-factor and Fanning friction factor f, as functions of Reynolds number based on louver pitch. The general correlations for j and f factors are developed and compared to other correlations. The f correlation indicates that the flow depth is one of the important parameters for the pressure drop.     

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re_cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.     

Air-side performance of brazed aluminum heat exchangers under dehumidifying conditions

An experimental study for air-side thermal-hydraulic performance of brazed aluminum heat exchangers under dehumidifying conditions has been performed. For 30 samples of louvered fin heat exchangers with different geometrical parameters, the heat transfer and pressure drop characteristics for wet surface were evaluated. The test was conducted for air-side Reynolds number in the range of 80–300 and tube-side water flow rate of 320 kg/h. The dry- and wet-bulb temperatures of the inlet air for heat exchangers were 27 and 19 °C, respectively and the inlet water temperature was 6 °C. The air-side thermal performance data for cooling and dehumidifying conditions were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluids unmixed. The test results are reported, compared with those for the dry surface heat exchangers, in terms of sensible j factor and friction factor f, as functions of Reynolds number based on louver pitch. The correlations for j and f factors are developed within rms errors of ±16.9 and ±13.6%, respectively.