Reduction of heat accumulated in a thermosyphon paddy bulk storage

The objective of this research work was to develop a thermosyphon paddy bulk storage for reducing heat accumulated due to paddy respiration. A prototype of thermosyphon paddy bulk storage and a mathematical model with lumped analysis for predicting the paddy bed temperature were conducted. The storage was a double-wall steel cylinder with 636 mm outside diameter and 1500 mm length. The evaporator section consisted of a set of 70 copper tubes with 3.0 m² surface area. The condenser section is the upper body of the storage tank and was ventilated with controlled air. In this study, the paddy bed, the ambient air, and methanol were used as the heat source, heat sink and working fluid in the thermosyphon, respectively.

The paddy bulk of 115 kg with 12.5% wet basis was placed in the evaporator section of the thermosyphon. From the experiment it was found that the paddy bed temperature could be reduced from 31–32 to 26–27°C within 15–20 hours. Moreover the actual overall heat transfer coefficient between the bed and the working fluid was evaluated to be about 4.3 W/m²°C. The paddy bed temperature could be maintained at about 26–27°C for eight weeks and the paddy qualities, whiteness and head yield, were in very good condition. In addition, the mathematical model developed could be used to predict the paddy bed temperature accurately.     

Cooling load reduction of building by seasonal nocturnal cooling water from thermosyphon heat pipe radiator

In this study, a concept of using thermosyphon heat pipe to extract heat from water in a storage tank to generate cooling water was proposed. Heat pipe condenser was attached with an aluminum plate and acted as a thermal radiator while its evaporator was dipped in the water storage tank. Cooling water in the tank could be produced during the nighttime and used to serve the cooling load in a room during the daytime. A heat transfer model to calculate the water temperature and the room temperature during both the nighttime and daytime was developed. The input data were ambient temperature, dew point temperature, area of the radiator, volume of cooling water and room cooling load. The experiment was setup to verify the heat transfer model. A 9.0 m² tested room with six cooling coils, each of 0.87 m² was installed at the ceiling, was constructed along with the 1.0 m³ water storage tank. A 500–2000 W adjustable heater was taken as an artificial load inside the room. A 6.36 m² radiator is installed on a 45° tilting roof of the tested room. The simulated results agreed very well with those of the experimental data. With the developed model, a simulation to find the sizing of the radiator area and the volume of cooling water for cooling water production during winter of Chiang Mai, Thailand was carried out. The cooling water was used for cooling during summer in an air‐conditioned room with different cooling loads. The parameters in terms of room temperature, radiator area, volume of cooling water, cooling load and UA of cooling coil were considered to carry out the percent of cooling load reduction. Copyright © 2009 John Wiley & Sons, Ltd.     

Heat and mass transfer in combined convective and far-infrared drying of fruit leather

Combined convective and far-infrared drying is a challenging assignment due to complex relationship between heat and mass transfer. In this paper, heat and mass transfer of fruit leather drying with combination of hot air and far-infrared has been carried out. The heat and the mass transfer coefficients were analyzed by heat–mass analogy. It could be found that the ratio between heat and mass transfer coefficients for the combination technique could not be obtained from the heat–mass analogy classical model and a modification is needed. The modified correlations for predicting ratio of heat and mass transfer coefficients and the heat transfer coefficient in term of heat transfer Nusselt number are developed. The model could fit the experimental data quite well within ±10% deviation.     

Effect of internal cooling/heating coil on adsorption/regeneration of solid desiccant tray for controlling air humidity

Thermal performances of solid desiccant tray having internal cooling/heating coil for air humidity adsorption and desiccant regeneration are investigated. Three units of desiccant tray each of 48 cm × 48 cm cross‐sectional area and 2.5 cm thickness filled with silica gel are tested in a wind tunnel. For adsorption process, an air stream is flowing through the desiccant trays and the air humidity is captured by the silica gel. Approximately 10–40% of air humidity could be adsorbed more in case of the internal cooling. Besides, the outlet air temperature increases only slightly. In regeneration process, a hot air stream is used to repel the moisture in the silica gel. With the internal heating, the regeneration time is shorter compared with that without internal water heating. In addition, a correlation for calculating the adsorption/regeneration performance of the silica gel trays is developed and the results from the model agree well with the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant

The aim of this paper is to simulate the performance of an air source heat pump water heater using carbon dioxide (CO₂) as a working fluid. The heat pump water heating system consists of a compressor, a gas cooler, an expansion device and an evaporator. The computer simulation model has been developed by using the heat transfer data and the thermodynamic properties of CO₂. The effects on the heat pump performance by the operating parameters such as the compressor rotational speed, the inlet water temperature at the gas cooler, the inlet air temperature at the evaporator and the mass flow rate ratio of water to refrigerant were presented. For rated capacities of a 4 kW compressor with a 10 kW gas cooler and a 6 kW evaporator, the coefficient of performance is found to be between 2.0 and 3.0. The mass flow rate ratio of water and CO₂ between 1.2 and 2.2 is the most suitable value for generating hot water temperature above 60°C at 15–25°C ambient air temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Ethyl alcohol distillation in a basin solar still

Prediction of alcohol distillation in a solar still with a horizontal evaporating surface and an inclined condensing surface was carried out, based on the model modified by Spalding [Convective Mass Transfer. Edward Arnold, 1963] and Kiatsiriroat et al. [Energy, 1986, 11, 881–886]. With the temperatures measured at the evaporating and condensing surfaces, including the concentration of alcohol in the liquid at the evaporating surface, the mass fluxes of concentrated ethanol, pure alcohol and water distillate yielded from the unit could be estimated. The predicted results agreed well with those of the experiments.     

Performance estimation of a PV water-pumping system with utilizability function

A procedure using the concept of the utilizability function for estimating the long-term performance of direct-coupled photovoltaic water-pumping systems is presented. A modified utilizability function, based on radiation statistics, is developed, and the monthly-averaged daily amount of water is simply estimated.     

Air-Cooling Enhancement with Delta Winglet Vortex Generators in Entrance Region of In-Line Array Electronic Modules

The objective of this study is to experimentally investigate the heat transfer enhancement by delta winglet vortex generators for air cooling in the entrance region of an in-line array of electronic modules. The study has been carried out when the winglet pairs are placed in front of all modules with attack angles of 10, 15 and 20 degrees. Each module has dimensions of 1.8 cm × 5.8 cm × 0.6 cm and generates heat at 5 W. The adiabatic heat transfer coefficients and the thermal wake functions for the modules with and without the generators are considered at different values of Reynolds number. It could be seen that the vortex generators could enhance the adiabatic heat transfer coefficients and reduce the thermal wake function and the module temperature significantly. Moreover, the correlations to predict the heat transfer data when the vortex generators are integrated have been developed. The temperatures of the modules in each row could be predicted from these correlations, and the results agree very well with the experimental data.     

Performance Analysis of Flat-Plate Solar Collector Having Silver Nanofluid as a Working Fluid

A study on water solar collector performance having silver nanofluid as working fluid was carried out. In this study, 20-nm silver particles mixed with water at the concentrations of 1,000 and 10,000 ppm were undertaken in 3 small identical closed-loop flat-plate solar collectors, each with an area of 0.15 m × 1.0 m. The mass flux of the working fluid varied between 0.8 and 1.2 L/min-m² and the inlet temperatures were controlled in the range of 35–65°C. The tests were performed outdoor under a steady-state condition. The experimental results showed that at the same Reynolds number, the convective heat transfer coefficient of the nanofluid inside the solar absorber tube at 1,000 ppm was slightly higher than that of water, and at 10,000 ppm, the heat transfer coefficient was about 2 times that of water. This meant that the overall heat loss coefficient of the solar collector with nanofluid could be reduced and more solar heat gain could be obtained, especially with a high inlet temperature of the working fluid. In our experiments, for 10,000 ppm concentration of silver nanoparticles, the optical characteristic and the thermal loss characteristic of the solar collector, under steady-state condition with a mass flux of 1.2 kg/min-m², were 0.691 and 4.869 W/m²-K, compared with 0.684 and 7.178 W/m²-K, respectively for 1,000 ppm concentration and 0.702 and 8.318 W/m²-K for water. When the flow rate was different from the standard value, the solar thermal characteristics were also improved with the nanofluid.     

Heat transfer model of slurry ice melting on external surface of helical coil

This research studies the heat transfer phenomenon of melting slurry ice on external surface of a copper helical coil. There is water flowing inside the tube coil and exchanging heat with the slurry ice. In this experiment, the coil diameters are 6.35 mm and 9.53 mm each of 4.2 m coil length. The mass flow rate of water in the helical coil is between 0.0149–0.0562 kg/s, while the inlet temperature of water is varied in the range of 23–27 °C. The slurry ice has 60% ice and 40% water by mass at the starting.