Experimental performance comparison and trade-off among air-based precooling methods for postharvest apples by comprehensive multiscale thermodynamic analyses

Air-based precooling methods including room cooling and forced-air cooling were traditionally used for postharvest horticultural products. In this study, disturbed-air cooling with different layouts was proposed for the trade-off between room cooling with long cooling time and forced-air cooling with high energy consumption. Lab-scale experiments with 30 bins of postharvest apples were conducted using the aforementioned methods to measure the temperature history. Multiscale thermodynamic analyses from energy, entropy, exergy, and entransy perspectives were then performed. The time evolution of transient quantities and overall comparison of the trade-off performances were further discussed. The ventilation power and transformed heat became more significant respectively for the total energy consumption and heat load during the precooling processes. The rates of entropy generation, exergy destruction, and entransy dissipation reduced in consistent with the tendency of heat rejection from all bins. The major part of these losses was resulted by the ventilation for convective heat transfer between cold air and apples and became more significant during later stage of precooling processes. The middle-parallel disturbed-air cooling achieved the best trade-off between the lowest energy consumption for room cooling and the lowest maximum seven-eighths cooling time for forced-air cooling by respectively reaching 81.68% and 28.82% of the optimization potential. The best trade-off between the lowest thermodynamic loss for room cooling and the lowest heat transfer ability loss for forced-air cooling was also achieved by this method with around 55% to 62% optimization of the coefficients of performance, around 83% optimization of the entropy generation ratio, around 58% to 62% optimization of the exergy destruction ratio, and around 36% optimization of the entransy dissipation ratio.     

Development of new correlations for forced convection heat transfer during cooling of products

This paper presents the new, simple but powerful effective Nusselt–Reynolds correlations for estimating the effective convective heat transfer coefficients of spherical and cylindrical products cooled in water and air flows. In this respect, both experimental and theoretical works were obtained. In the experimental case, several spherical and cylindrical products, namely, tomatoes, pears and cucumbers were cooled in water and air flow and their centre temperature variations were measured. In the theoretical case, the effective convective heat transfer coefficients for the individual spherical and cylindrical products were determined using the centre temperature data in the present approach including Dincer's models. Therefore, the new Nusselt–Reynolds correlations were developed using the effective convective heat transfer coefficient values and a general diagram of Nu/Pr^1/3 against Reynolds number was drawn. This study indicates that the present effective Nu–Re correlations are capable of estimating the effective convective heat transfer coefficients of any spherical and cylindrical shaped products exposed to water and air cooling in practical applications in a simple and accurate manner.     

A simple solution for transient heat transfer during heating of a slab product

A theoretical and experimental study of transient heat transfer in the heating of an individual slab product, subjected to an air flow at a temperature of 50°C and a velocity of 1 m/s, is presented. Experimental temperature measurements at the centre of the slab product were made, and the experimental heat-transfer rates were derived from the temperature data. A simplified analytical technique, using the boundary condition of the third kind in transient heat transfer, was used to predict the theoretical heat transfer rates for two cases, the first considering that the heat transfer coefficient is a convective heat transfer coefficient, and the second considering that heat transfer coefficient is the sum of the convective and radiative heat transfer coefficients. The experimental heat-transfer rates were compared with the predictions for two cases, and a very good agreement was obtained.     

Transient temperature distributions in canned products during heat sterilization and cooling

This reseach was performed using experimental temperature data and a simple analytical model to estimate the temperature distributions at the centres of cylindrically canned products (diameter/thickness ratio = 4/1) during heat sterilization and cooling under continuous flow conditions for two retort temperatures (115 and 121°C). The recorded experimental temperature data for an individual can for two different retort temperatures were then used to construct the dimensionless experimental centre temperature curves. In order to predict dimensionless centre temperature profiles with a mathematical model, the convection boundary condition (i.e. 0·1 < Bi < 100) in transient heat transfer was considered. The predicted temperature profiles were compared with the experimental centre temperature measurements for two cases—heat sterilization and cooling. The experimental temperature values were in very good agreement with the theoretical predictions. The results of this study show that the present technique is a reasonable tool for estimating in a simple and accurate form the temperature distributions of a cylindrically canned product subject to both heat sterilization and cooling.     

Correlation analysis of transient heat transfer characteristics for air precooling aggregate

In dam works,air precooling of aggregate is a common and effective method to avoid temperature cracks in concrete structure.In order to offer a reliable design theory for the air precooling process to avoid unreasonable energy consumption,the transient heat transfer characteristics of the aggregate are intensively analyzed.The combined structure of the aggregate and the interstitial space in the hopper is treated as a porous structure,and the space-average method is used to simulate the transient heat transfer process.Simulation results show that size of the hopper and the average air velocity in the cross section have great influence on the transient heat transfer process of the aggregate,while the porosity in the range of 0.4 0.5 has little influence.Nomograms are abstracted from simulation results,and then correlations of the compared excess temperature are precisely fitted to predict the air precooling transient heat transfer process of the aggregate.     

Convective heat transfer coefficients evaluation for a portable forced air tunnel

The objective of this work was the determination of the convective heat transfer coefficient in order to evaluate an experimental portable forced-air freezing tunnel and work on comparative studies with air exhausting and blowing. The heat transfer coefficients of the cooling air and the product were analyzed during freezing process. Convective coefficient results were higher for air exhaustion than air blowing in every part of the batch, except for the upper layer of products, where the cooling air of the chamber was directly in contact with the product. These results, with the temperature analysis obtained, indicated that the air circulation around the samples, as well as the heat transfer, improved with the use of the portable system, and had better results for the exhaustion compared to the blowing process.     

Simplified solution for temperature distributions of spherical and cylindrical products during rapid air cooling

The transient heat transfer between individual spherical and cylindrical products and the air medium during cooling was analyzed and modeled, and therefore, the center temperature distributions of the spherical and cylindrical products were computed in a simple manner. These computed temperature distributions were compared with experimental measurements, and very good agreement was found between the computed and measured temperature values.     

An effective method for analysing precooling process parameters

An investigation of the precooling process parameters of the individual fruits (tomatoes, cucumbers, squashes, and eggplants) in the batches of 5, 10, 15, and 20 kg subjected to hydrocooling is presented. Experimentally measured centre temperatures of the fruits were used in the analysis, which was based on the unsteady-state heat transfer, and the process parameters were determined. As a result, the precooling process parameters in terms of the cooling coefficient, lag factor, and half cooling time were found to be dependent upon the batch weight of the product. The results confirmed the effectiveness of the present method to determine precooling process parameters with a reasonable accuracy.     

Cooling of spherical products: Part II—heat transfer parameters

This paper presents a study on the determination of the heat transfer parameters, namely surface heat transfer coefficients, thermal conductivities, thermal diffusivities, specific heats and Biot numbers, for the individual product being cooled with water and with air. An analytical model was developed to determine the surface heat transfer coefficients of the products depending on the thermal properties and cooling process parameters. The results of the present study indicate that surface heat transfer coefficients decrease with increasing batch weight in water cooling and increase with increasing air flow velocity in air cooling. The proposed model can be used to determine easily and accurately surface heat transfer coefficients of different spherically shaped objects subjected to cooling.     

New correlations for heat transfer coefficients during direct cooling of products

An analytical method for determining the heat transfer coefficients of food products being cooled in water and in air flows is presented. Food products are idealized as geometrical solid objects of regular shapes. New correlations between heat transfer coefficients and cooling coefficients are developed in simple forms for practical use in the refrigeration industry. These correlations are then used to determine the heat transfer coefficient for a cylindrical carrot cooled in air flow as an illustrative example. In addition, evaluating the heat transfer coefficients for several products using the available experimental cooling coefficient values from the literature, two new correlations between the heat transfer coefficient and the cooling coefficient are also obtained for water and air cooling applications. The results show that the correlations presented in this article can determine the heat transfer coefficients of food products forced-convection cooling in a simple and accurate manner.     

Unsteady conjugate laminar heat transfer of a rotating non-uniformly heated disk: Application to the transient experimental technique

Presented in the paper are the results of an investigation of 2D heat conduction effects on the transient heat transfer of a rotating disk heated up to a non-uniform initial temperature and suddenly subjected to unsteady cooling by still air. A self-similar solution of the transient laminar convective heat transfer confirmed that the heat transfer coefficient rapidly becomes time-independent and equal to its value at steady-state conditions. An analytical solution of the unsteady two-dimensional heat conduction inside a disk made of Plexiglas^® confirmed that the known infinite-slab approach can still be used as a transient technique for determining heat transfer coefficients. Use of the regular heat transfer regime theory for the same purpose can be recommended only for the cases with the moderate initial temperature non-uniformity.     

Improvement of the pulsed photothermal technique for the measurement of the convective heat transfer coefficient

<正>The present study concerns the measurement of the convective heat transfer coefficient on the solid-fluid interface by the pulsed photothermal method.This non-intrusive technique is apphed for the measurement of the local heat transfer coefficients in cooling of a rectangular slab that simulates an electronic component.The heat transfer coefficient is deduced from the evolution of the transient temperature induced by a sudden deposit of a luminous energy on the front face of the slab.In order to draw up the heat transfer cartography by a non-destructive tool, the infrared thermography has been used.Two inverse techniques for the identification of the heat transfer coefficient are presented here.The first one is based on the assumption that heat transfer coefficient remains constant during the pulsed experiment,and the second one considered it variable in space and time.The temporal and spatial evolutions are expressed as a constant heat transfer coefficient(h_0)multiplied by a function of time and space f(x,t).The function f is deduced from the resolution of the conjugated convection-conduction problem,by a control volume technique for the case of thermally thick sample.The results are given for different air velocities and deflection angles of the flow.     

Development of a new number (the Dincer number) for forced-convection heat transfer in heating and cooling applications

In this article, a new number is developed for the use of forced-convection transient heat transfer to take place during heating and cooling of any solid object. This is called the ‘Dincer number’ which indicates the effect of the flow velocity of the surrounding fluid on the heating coefficient or cooling coefficient. It indicates that there is a strong relation between the Nusselt number and the Dincer number. For this reason, an application of this number was made and a new Nusselt-Dincer correlation, namely Nu = (2·2893 × 10 ⁻⁴ Di^1·0047) was developed for food products exposed to forced-air cooling using the experimental data sets. It is therefore possible to determine the heat transfer coefficient from the Nusselt number after estimating the Dincer number. It can be concluded that the Dincer number brings a new approach to heating and cooling problems. Owing to their simplicity and effectiveness, development of the Nusselt-Dincer correlations will be beneficial to scientists and industries concerned with heating and cooling applications.     

Analytical modelling of heat transfer from a single slab in freezing

The present study deals with the experimental and theoretical analysis of the transient heat transfer from an individual slab body to the medium during freezing. In this respect, a new model was developed to determine the heat transfer coefficient and this heat transfer coefficient was used in the computation of the dimensionless theoretical temperature distribution. On the other hand, experimental work was performed to measure the centre temperatures of the individual slab products (viz. dried figs) during freezing at the median temperature of -22°C. In the comparison of the theoretical and experimental temperature distributions, very good agreement was found.     

Total heat transfer coefficients for canned foods during sterilization

An analytical mathematical model for determining the total heat transfer coefficient of a cylindrically shaped canned food subjected to sterilization was developed. There is a need to determine these coefficients in a simple and accurate form for process heat transfer analysis and energy optimization. In the mathematical modelling, a new technique for heat sterilization conduction problem was used by considering the boundary condition of the third kind in transient heat conduction. The temperature data at the centres of the cylindrically shaped cans were obtained in the experimental investigation at the medium temperatures of 115 and 121°C and were used to determine the total heat transfer coefficients of the individual canned products. The total heat transfer coefficient for an individual canned product increased with increasing medium temperature. The results of this study shows that the present analytical model is a simple tool for determining the total heat transfer coefficients for the individual canned products.     

A simple and accurate method of measuring surface film conductance for spherical bodies

An empirical relation has been developed for Biot number, which enables the calculation of surface film conductance during the transient cooling of spherical bodies by measuring temperature at any location inside the body and the total elapsed time. The method was used to measure heat transfer coefficient of apples, oranges and potatoes through precooling experiments. The results have been compared with those obtained by the Nu-Re relationship available in the literature. The values determined by the present method yield excellent time-temperature calculations.     

Experimental Study of Heat Transfer in Gas Turbine Blades Using a Transient Inverse Technique

To enhance specific power output and thermal efficiency of gas turbine engines, industry searches for ways to increase the turbine inlet temperatures. Therefore, temperatures of turbine blades increase as well and necessitate active cooling of these components. Experimental design work on such internal cooling schemes is carried out to find acceptable compromises between heat transfer and pressure losses. It is often carried out by using transient thermochromic liquid crystal techniques in combination with Plexiglas models. However, for real turbine blades this experimental technique is inappropriate due to the lack of optical access. Therefore, to study actual turbine blades there is need for development of noninvasive, nondestructive methodologies. This article describes a measurement technique that allows determination of internal heat transfer coefficients of real turbine blades experimentally. Thus, a test rig with a rapidly responding heater was designed to fulfill the requirement of a sudden increase in the air temperature within the cooling passages. The outer surface temperatures were measured using infrared thermography. To estimate the spatial distribution of internal heat transfer coefficients from transient surface temperatures the inverse heat transfer problem was solved. As optimization algorithm the Levenberg–Marquardt method was chosen. Outer surface temperature data was measured for a rectangular reference model with rib turbulators and compared with simultaneously acquired data using the thermochromic liquid crystal technique. It is concluded that the new experimental measurement technique could be used to quantitatively determine internal heat transfer coefficients.     

果蔬预冷冰浆式湿冷热湿交换器性能实验

针对果蔬预冷设备应用场合,提出并设计了一套以冰浆作为载冷介质的湿冷热湿交换器,并搭建单体性能测试台架,以出风温度和相对湿度为指标,通过改变填料类型(金属、纸质填料)、载冷介质种类(冰浆、冷水)和喷淋流量进行了性能实验研究。结果表明:实验工况下,金属填料的换热性能较纸质填料好;以冰浆作为载冷介质相比以冷水的情况,可以获得更低的出风温度,但出风相对湿度也有所降低;随着进风干球温度的降低,出风温度明显降低,而出风相对湿度变化并不明显;在一定范围内,提高载冷介质的喷淋流量,有利于湿冷热湿交换器出风温度的降低和出风相对湿度的升高;低浓度的冰浆可以在湿冷热湿交换器中稳定运行,且降温效果较冷水湿冷热湿交换器更加明显,虽然相对湿度略有下降但仍然可保持在90%左右,适用于果蔬预冷和保鲜。     

The use of phase-change coatings for estimating the heat transfer characteristics in a rotating disc system

Phase-change coatings have been applied to the axial-clearance rotor-stator cavity for the estimation of the transient heat transfer characteristics of the surface of the rotating disc. The tests were conducted for an air mass flow coefficient C_w = 1220, a gap ratio G = 0.1, an axial-clearance ratio G_ca = 0.05 and for rotational Reynolds numbers of Re? = 1 × 10⁵ and 2 × 10⁵. The phase-change coating used had a melting point of 38°C. From a video recording of the transient movement of the melt-line on the rotor (coated with the phase-change material) blown with heated air, it was possible to compute the heat transfer coefficients. The data reduction was made using the ‘semi-infinite slab’ approximation to the governing one-dimensional transient heat conduction equation.     

Experimental evaluation of local instantaneous heat transfer characteristics in the combustion chamber of air-cooled direct injection diesel engine

An experimental analysis is conducted investigating the differences between the variations of overall and local instantaneous heat transfer coefficients, during the engine cycle, in the combustion chamber walls of a direct injection (DI), air-cooled diesel engine located at the authors’ laboratory. For this purpose, a novel experimental installation is developed, which separates the engine transient temperature signals into two parts, namely the long- and the short-term response ones, processed in two independent data acquisition systems. Moreover, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an object-oriented control code for fast data acquisition have been designed and applied. Experimentally obtained cylinder pressure diagrams are used as a basis for the calculation of the overall heat transfer coefficients, whereas one-dimensional heat conduction theory with Fourier analysis techniques, combined with an iterative procedure between calculated and measured temperature data, are implemented in order to calculate the instantaneous local heat transfer coefficients in the engine cylinder. Analysis of the experimental results reveals interesting aspects of transient engine heat transfer. Significant differences are disclosed between the overall and local heat transfer coefficient variations, with the importance of the latter one on engine design being emphasized. The local heat transfer coefficient on the cylinder head is quantified based on the experimental data. The effect of engine speed and load as well as of the air swirling motion on the heat transfer variations are presented. From the analysis results it is concluded that the instantaneous heat transfer variation is non-uniform, unlike its values calculated from standard correlations that assume spatial uniformity, noting that such information, especially for air-cooled diesel engines, seems to be very scarce in the open literature.