J. Tiansuwan

Optimization of heat pump fruit dryers

The most important factors when examining the optimum conditions of heat pump fruit dryers (HPD) and for minimizing HPD cost are recycle air ratio, evaporator bypass air ratio (BP), airflow rates, and drying air temperature. Mathematical models of papaya and mango glace drying using HPD are developed and validated experimentally. The optimum criterion is minimum annual total cost per unit of evaporating-water. The effects of initial moisture content, cubic size, and effective diffusion coefficient of products on the optimum conditions of HPD are also investigated. The results showed that the optimum conditions of both products are not similar, especially for the optimum airflow rate and evaporator BP. For sensitivity analysis, the annual total cost per unit of evaporating-water in a HPD is linearly proportional to both interest rate and electricity price, and decreased with increasing life-time. Finally, physical properties of the product significantly affect the optimum airflow rate and evaporator bypass air ratio.

Thermal Performance Enhancement of Thermosyphon Heat Pipe with Binary Working Fluids

Thermal performance of a thermosyphon heat pipe using ethanol-water and TEG-water with variations of parameters such as the mixture content, the pipe diameter, and the working temperature have been studied in this research work. From the experiments, it is found that at low temperature of heat source (less than 80oC), the ethanol-water mixture has a higher heat transfer rate than that of water and close to that of pure ethanol. In the case of TEG-water mixture, the heat transfer rate of the thermosyphon varies with the content of TEG in the mixture, and it is found that TEG in the mixture can increase the critical heat flux due to the flooding limit in a small thermosyphon. The boiling equation of Rohsenow and the condensation equation of Nusselt are modified to predict the heat transfer coefficients inside the thermosyphon. For the mixtures, the weighted average of the heat transfer coefficient of each component can be used to predict the total heat transfer coefficient. Furthermore, it is found that Fag...Thermal performance of a thermosyphon heat pipe using ethanol-water and TEG-water with variations of parameters such as the mixture content, the pipe diameter, and the working temperature have been studied in this research work. From the experiments, it is found that at low temperature of heat source (less than 80oC), the ethanol-water mixture has a higher heat transfer rate than that of water and close to that of pure ethanol. In the case of TEG-water mixture, the heat transfer rate of the thermosyphon varies with the content of TEG in the mixture, and it is found that TEG in the mixture can increase the critical heat flux due to the flooding limit in a small thermosyphon. The boiling equation of Rohsenow and the condensation equation of Nusselt are modified to predict the heat transfer coefficients inside the thermosyphon. For the mixtures, the weighted average of the heat transfer coefficient of each component can be used to predict the total heat transfer coefficient. Furthermore, it is found that Faghris equation can be used to predict the critical heat flux due to the flooding limit of the thermosyphon with the binary mixtures.

Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: A case study of TEG–water

Abstract In this research, the critical heat flux (CHF) due to flooding limit of thermosyphon heat pipe using triethylene glycol (TEG)–water mixture has been investigated. From the experiment it is found that, use of TEG–water mixture can extend the heat transport limitation compared with pure water and higher heat transfer is obtained compared with pure TEG at high temperature applications. Moreover it is found that ESDU equation is appropriate to predict the CHF of the thermosyphon in case of TEG–water mixture. For thermosyphon air preheater at high temperature applications, it is found that with selected mixture content of TEG–water in each row of the thermosyphon the performance of the system could be increased approximately 30–80% compared with pure TEG for parallel flow and 60–115% for counter flow configurations. The performances also increase approximately 80–160% for parallel flow and 140–220% for counter flow compared with those of pure dowtherm A which is the common working fluid at high temperature applications.

Combustion of blends between plant oils and diesel oil

Combustion rates of blends between plant oils and diesel oil were predicted by a convective mass transfer theory. Test results for the combustion rates of diesel oil blended with palm oil, soy bean oil and rice bran oil, on a spherical model of 50 mm diameter, agree well with the predicted values.

STUDY OF LONGAN FLESH DRYING

The objectives of this research were to study the characteristics and quality of longan flesh drying and to develop the models for drying simulations. Finite difference method was applied to solve the drying kinetic equations. Three alternative diffusion models were developed by modifying the Arrhenius factor and/or the energy of activation as a function of moisture content. The results using effective diffusion coefficients obtained from each model were compared. Desorption isotherms were also developed by fitting experimental results to various well-known models. The development and simulation of mathematical drying model of a cabinet dryer were also studied. The effects of drying air temperature and specific air flow rate on specific energy consumption were described. Additionally, the effect of drying air temperature on product quality was investigated by experiment.

Performance analysis of thermosyphon heat exchanger under electric field

Abstract This paper presents a numerical method to analyze the thermosyphon heat exchanger with and without the presence of electrohydrodynamics (EHD). The proposed model is capable of handling both balanced and unbalanced thermosyphon heat exchangers. For the balanced thermosyphon heat exchanger, the calculated results of heat transfer rate for water and R-134a agree well with experimental data. For the unbalanced thermosyphon heat exchangers, it is found that the performance improvement increases with the ratio of m e / m c when EHD is applied at the condenser alone. Conversely, the performance improvement decreases with the ratio of m e / m c when EHD is applied at the evaporator alone.

Performance analysis of a direct-contact thermal energy storage-solidification

Performance of a direct-contact latent heat energy storage during discharging process has been investigated. The storage medium used is sodium thiosulphate pentahydrate of which the melting temperature is 48°C and the heat exchanging fluid is heat transfer oil. An empirical expression to evaluate the volumetric heat transfer coefficient has been carried out. A lumped analysis is also found to be quite suitable to analyze the temperature of the storage medium and its solid fraction including the temperature of the oil leaving the storage. The results agree well with those of the experiments.

Energy optimisation of whole longan drying: simulation results

SYNOPSIS The objective of this work is to determine the optimal operating conditions for the process of longan drying. The evaluation criteria of the process are the specific energy consumption and the drying time. Mathematical models were developed to simulate the longan drying process. The models are verified by comparison between the predicted results and experimental measurements. Comparisons between the predicted results and the measurements showed good agreement. The models were used to evaluate the effects of the operating parameters on the specific energy consumption. The operating parameters are drying air temperature, specific airflow rate, and the fraction of recycled air. At the optimal operating conditions, the specific energy consumption was determined to be 4.2 MJ/kg-water.

Performance improvement of thermosyphon heat exchangers by using two kinds of working fluids

In this study, the concept of introducing two-fluid thermosyphons is examined. Calculations were performed for both low and high temperature ranges with parallel and counter-flow arrangements. For lower-temperature application, 125°C > T hi > 75°C, use of ammonia in some rows and water in the rest of the thermosyphon can slightly improve the associated heat transfer performance for balanced counter-flow arrangement. However, for balanced parallel-flow arrangement in both low-and high-temperature applications, the concept of using two-fluid thermosyphons may not be feasible. The use of two-fluid thermosyphons is especially advantageous for high-temperature applications. For instance, in the range of 375°C > T hi > 350°C, the two-fluid thermosyphons (Dowtherm A-water) shows a 15-99% increase of heat transfer performance relative to Dowtherm A alone.In this study, the concept of introducing two-fluid thermosyphons is examined. Calculations were performed for both low and high temperature ranges with parallel and counter-flow arrangements. For lower-temperature application, 125°C > T hi > 75°C, use of ammonia in some rows and water in the rest of the thermosyphon can slightly improve the associated heat transfer performance for balanced counter-flow arrangement. However, for balanced parallel-flow arrangement in both lowand high-temperature applications, the concept of using two-fluid thermosyphons may not be feasible. The use of two-fluid thermosyphons is especially advantageous for high-temperature applications. For instance, in the range of 375°C > T hi > 350°C, the two-fluid thermosyphons (Dowtherm A-water) shows a 15-99% increase of heat transfer performance relative to Dowtherm A alone.

Effect of anti-solar glass film on heat transfer and mean radiant temperature inside cabin of air-conditioned vehicle

SYNOPSIS This paper addresses solar heat load entering an air-conditioned pickup-car cabin and behaviour of the mean radiant temperature, MRT, in the cabin including the fuel consumption due to the air conditioner. The experiments have been carried out under the climate of Bangkok, Thailand when all the windows except the windscreen are coated with 40% cutoff anti-solar film. A set of mathematical models to evaluate the surface temperatures of the windows and the cabin walls has been presented and the results agree quite well with those of the experimental data. With the temperatures inside the car cabin, the MRT could be calculated. It was found that with the film coating on the windows, the MRT inside the cabin comes closer to the inside air temperature than without the film. For films of 40, 60 and 80% radiation cutoff, when the outside air temperature is lower than 25°C, the lighter film gives similar performance to the darker film. When the outside temperature is higher than 25°C, the darker film shows an advantage in reducing the MRT. In a car with 40, 60 and 80% cutoff anti-solar film the cooling load could be reduced by about 14, 18 and 20%, respectively, compared with that without the film. Fuel savings could be about 11.7, 14.4 and 18%, respectively.