Periodic theory of an open roof pond

This paper describes an analysis of the reduction of the heat flux coming into a room through a roof with an open water pond. A periodic analysis of the system, based on the linearisation of Dunkle's⁶ expressions for convective, radiative and evaporative losses, is presented. Numerical computation for the heat flux has been carried out for a typical hot day, the 19th of June, 1979, in New Delhi. These results were compared with those for a water-sprayed system. It was found that, for typical parameters, there are reductions in the maximum heat flux entering the room of 48% and 41% for the roof pond and water-sprayed systems, respectively (for a relative humidity of 0·8). The corresponding reductions in daily heat input into the room are 20% and 35%, respectively. Load levelling is, of course, much better in the case of the open roof pond.     

Comparative thermal performance evaluation of an active solar distillation system

In this paper, thermal models of all types of solar collector‐integrated active solar stills are developed based on basic energy balance equations in terms of inner and outer glass temperatures. In this paper, hourly yield, hourly exergy efficiency, and hourly overall thermal efficiency of active solar stills are evaluated for 0.05 m water depth. All numerical computations had been performed for a typical day in the month of 07 December 2005 for the climatic conditions of New Delhi (28°35′N, 77°12′E, 216 m above MSL). The thermal model of flat‐plate collector integrated with active solar still was validated using the experimental test set‐up results. Total daily yield from active solar still integrated with evacuated tube collector with heat pipe is 4.24 kg m^?2 day^?1, maximum among all other types of active solar stills. Copyright © 2007 John Wiley & Sons, Ltd.     

Solar distillation under climatic conditions of Egypt

Solar distillation in a single basin was studied theoretically under the climatic conditions of Alexandria, Egypt. The unsteady energy equations for the glass cover, water and basin were simultaneously solved. The effects of saline water depth, insulation thickness and wind speed on the still productivity were evaluated. A year-round study showed that the productivity of the still strongly depends on the solar radiation and ambient temperature. The daily still productivity varies from 1.1 to 5.2 kg/m² of basin area with an annual average of 3.16 kg/m². The solar still efficiency variesfrom 0.34 to 0.49 with an annual average of 0.42. The daily total still production increased with decreasing the water depth and increasing the insulation thickness. Increasing the wind speed resulted in a relatively small reduction in still productivity. The maximum production rate occurred after the peak in solar radiation and the time lag increases with the increase in water depth.     

Thermal analysis of constant flow partitioned solar pond

This paper presents the thermal analysis of the process of heat extraction by circulating water layer through the convective zone of a partitioned solar pond. The observed variation of atmospheric air temperature and solar intensity is assumed periodic. Explicit expressions for the transient rate and temperature at which heat can be extracted by circulation of water at constant flow rate, are derived. Numerical computations corresponding to solar heat flux and atmospheric air temperature measurement at New Delhi during the year 1974 have been made, and the optimization of the flow rate as well as the depth of the convective-non-convective zones in the pond have been investigated. The optimum heat retrieval efficiency of 27.5%, 34% and 40% corresponding to heat retrieval temperatures of 97°C, 60.5°C and 45.5°C, respectively, are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s.m², respectively. The load levelling in retrieved heat flux improves as flow rates are lowered, and the non-convective zone is oversized. With the non-convective zone depth near optimum, an increase in the depth of the heat extraction zone considerably influences the retrieved heat flux; it shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the extraction zone and larger depths of non-convective zone.     

Characteristic of local boiling heat transfer of ammonia and ammonia / water binary mixture on the plate type evaporator

Power generation using small temperature difference such as ocean thermal energy conversion (OTEC) and discharged thermal energy conversion (DTEC) is expected to be the countermeasures against global warming problem. As ammonia and ammonia/water are used in evaporators for OTEC and DTEC as working fluids, the research of their local boiling heat transfer is important for improvement of the power generation efficiency. Measurements of local boiling heat transfer coefficients were performed for ammonia /water mixture (z = 0.9−1) on a vertical flat plate heat exchanger in a range of mass flux (7.5–15 kg/m² s), heat flux (15–23 kW/m²), and pressure (0.7–0.9 MPa). The result shows that in the case of ammonia /water mixture, the local heat transfer coefficients increase with an increase of mass flux and composition of ammonia, and decrease with an increase of heat flux.     

A General Predictive Technique for Heat Transfer during Saturated Film Boiling in Tubes

Abstract

A simple predictive technique for heat transfer during film boiling in tubes is presented. This technique is based on the two-step model and consists of a graphic correlation for nonequilibrium quality and an equation for liquid droplet cooling at high pressures. It has been developed from and verified with data for water, nitrogen, para-hydrogen, R-113, methane, and propane. The range of data includes equilibrium qualities from 0.1 to 2.9, pressures from 1.4 to 215 bar, reduced pressures from 0.01 to 0.97, mass flux from 30 to 3442 kg/m² s, tube diameters from 2.5 to 14.9 mm, heat flux from 0.012 to 2.1 [Macute]W/m², and wall temperatures from 81 to 1112 K. For all 722 data points analyzed, heat transfer coefficients based on actual vapor temperatures are correlated with a root-mean-square error of 15%.     

TWO-PHASE PRESSURE DROP OF CO2 IN MINI TUBES AND MICROCHANNELS

Two-phase pressure drops of CO₂ are investigated in mini tubes with inner diameters of 2.0 and 0.98 mm and in microchannels with hydraulic diameters from 1.08 to 1.54 mm. For the mini tubes, the tests were conducted with a variation of mass flux from 500 to 3570 kg/m²s, heat flux from 7 to 48 kW/m², while maintaining saturation temperatures at 0, 5, and 10°C. For the microchannels, mass flux was varied from 100 to 400 kg/m²s and heat flux was altered from 5 to 20 kW/m². A direct heating method was used to provide heat flux into the refrigerant. The pressure drop of CO₂ in the mini tubes shows very similar trends with those in large diameter tubes. Although the microchannel has a small hydraulic diameter, it shows a larger Chisholm parameter in two-phase multiplier. Based on the experimental data in this study, the Chisholm parameter in the Lockhart and Martinelli correlation is modified by considering diameter effects on the two-phase multiplier.     

Experimental study of water vapour condensation on a rotating disc

Condensation of saturated low pressure water vapour on the bottom side of a smooth surface horizontal aluminium spinning disc was studied experimentally. Results of the heat flux on the disc as a function of the wall superheat and the speed of rotation show that the mean heat transfer coefficient varies from 10 kW/m²K at 0 rpm to 30 kW/m²K at 1000 rpm in the low heat flux range and from 6 kW/m²K at 0 rpm to 20 kW/m²K at 1000 rpm in the high heat flux range. The local heat transfer coefficient does not change with radius.     

Thermal load levelling in a multilayered wall/roof

An explicit expression for the periodic variation of thermal flux through a multilayered insulated hollow wall/roof, whose one face is exposed to solar radiation and ambient air and the other is in contact with room air at constant temperature, has been derived. to obtain optimum placement of insulation and airgap, numerical calculations for the heat flux through a multilayered insulated hollow wall/roof have been made for a typical hot day (26 May 1978) in Delhi. It is seen that for a given total thickness of concrete, best load levelling is achieved when the thickness of the outer layer is as small as possible. the effect of a water film on heat flux into the buidling has also been discussed.     

Composite pin-fin heat sink for effective hotspot reduction

This current work aims to decrease temperature nonuniformity in a microprocessor. The proposed composite pin-fin heat sink design is analyzed computationally, and its functioning is compared with the conventional heat sink design. According to the heat rate, the composite heat sink is divided into two sections: the hotspot and the background section. Aluminum, copper, and graphene are chosen for the background and hotspot sections. Both noncomposite and composite heat sinks are designed with similar geometrical dimensions. DI water is used as the working fluid. They are studied for heterogeneous hotspot heat flux varying from 200 to 600 kW/m² by keeping constant background heat flux as 100 kW/m² with the inlet mass flow rate of 0.05 kg/s. Further simulations are performed for various Reynolds numbers (Re = 150, 225, 300) with a constant background and hotspot heat flux of 100 and 600 kW/m², respectively, for different inlet temperatures of 15°C, 20°C, and 25°C. The simulations are also carried out for other working fluids, such as TiO₂ and Fe₂O₃ based nanofluids with the constant volume concentration of 0.65% and 3%, respectively in the DI water, at the constant background and hotspot heat flux of 100 and 600 kW/m², respectively. The results are shown for all the above studies planned. The results suggest that composite heat sinks with graphene as a composite material and Fe₂O₃ based nanofluid yields higher heat dissipation.     

Geothermal assessment of the deep aquifers of the northwestern part of the Bohemian Cretaceous basin, Czech Republic

Groundwater in the Benešov-Ústí aquifer system in the northwestern Bohemian Cretaceous basin has been intensely exploited since the twentieth century. Apart from providing drinking water, it contains the most extensive accumulation of thermal water in the country. However, excessive exploitation can result in temperature declines and changes in the quality of the groundwater in the future. More than a hundred in situ temperature measurements were used to assess the geothermal gradient and heat flux. However, intense groundwater vertical flow across the well significantly controls the heat flux distribution, resulting in a huge range of values—from less than 50 mW/m² within infiltration areas to more than 125 mW/m² in drainage areas. Certain simplifications and corrections considering the vertical flow between different permeable zones were developed, and the correction for topography as well as lithological variability have been applied to improve accuracy of the geothermal gradient assessment. Despite the fact that the Bohemian Cretaceous basin is tectonically very complex, it is concluded that tectonics [with the exception of the Eger (Oh?e) rift] has only a secondary effect on the thermal field. Two longitudinal W-E areas in the Benešov-Ústí aquifer system have elevated heat flux values. The calculated heat flux values are useful for heat transfer modelling and the assessment of the sustainable limits of thermal water exploitation.     

Thermal management of solar cells using a nano‐coated heat pipe plate: an indoor experimental study

With temperature increasing, the photovoltaic efficiency of solar cells is reduced significantly. Such an efficiency loss may offset the efficiency improvement because of the development of the photovoltaic technology. This paper provides a novel approach for efficiency loss recovery of solar cells. Specifically, a nano‐coated heat pipe plate was integrated with the solar panel to remove heat from the hotspots on solar cells. This study concerns the indoor experiments of a commercial solar cell thermally managed with a heat pipe plate. The temperature rise and non‐uniformity on the solar panel were quantified in different light irradiances. With thermal management by the heat pipe plate, the solar panel shows a temperature‐rise reduction of 47–50%. This implies that half of the efficiency loss of the solar cell can be recovered. In addition, the temperature variation within the solar panel is reduced to 1.0–2.5 °C, which is beneficial in prolonging the longevity of the solar cell. In the experiments, the heat pipe plate can provide a cooling flux of 380 W/m² with light irradiance below 1000 W/m². By incorporating the heat pipe plate with a water jacket, the heat removal flux could be improved to 600 W/m², leading to a solar cell temperature of a few degrees higher than the ambient. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental Study on Flow Boiling of Deionized Water in a Horizontal Long Small Channel

In this paper, an experimental investigation on the flow boiling heat transfer in a horizontal long mini-channel was carried out. The mini-channel was with 2 mm wide and 1 mm deep and 900 mm long. The material of the mini-channel was stainless. The working fluid was deionized water. The experiments were conducted with the conditions of inlet pressure in the range of 0.2~0.5 MPa, mass flux in the range of 196.57-548.96 kg/m²s, and the outlet vapor quality in the range of 0.2 to 1. The heat flux was in the range of 292.86 kW/m² to 788.48 kW/m², respectively. The influences of mass flux and heat flux were studied. At a certain mass flow rate, the local heat transfer coefficient increased with the increase of the heat flux. If dry-out occurred in the mini-channel, the heat transfer coefficient decreased. At the same heat flux, the local heat transfer coefficient would depend on the mass flux. It would increase with the mass flux in a certain range, and then decrease if the mass flux was beyond this range. Experimental data were compared with the results of previous studies. Flow visualization and measurements were conducted to identify flow regime transitions. Results showed that there were eight different kinds of flow patterns occurring during the flow boiling. It was found that flow pattern had a significant effect on heat transfer.     

Convective boiling heat transfer characteristics of CO2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO₂ are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO₂ from 200 to 400 kg/m² s, heat flux from 10 to 20 kW/m², while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO₂ is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m² and a mass flux of 300 kg/m² s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.     

Experimental and numerical study on heat transfer and flow characteristics in an alternating cross‐section flattened tube

An experimental and numerical study on convection heat transfer of water flowing through an alternating cross‐section flattened (ACF) tube are investigated in this paper. The thermal‐fluid characteristics were evaluated by numerical simulation. The test run conditions covered a mass flux of 200 to 800 kg m^?2 s^?1, a heat flux of 10 kW/m², and an inlet temperature of 40°C. The results showed that the Nusselt number increased with the increase in mass flux. Moreover, the heat transfer was also affected by the flow characteristics. Vortices were formed at the curved wall, and their intensities were increased along the flow direction. It was also found that the heat transfer and pressure drop were larger than that of the circular tube. However, the thermal performance was greater than the pressure loss penalty. The comparison results showed that the ACF tube had better performance than the circular tube. Further, the details of heat transfer, flow resistance, and fluid behavior were investigated and discussed in this study.     

Effects of surfactant additive on flow boiling over a microheater under pulse heating

An experimental investigation has been carried out to study effects of surfactant additive on microscale boiling under pulse heating over a Pt microheater (140 × 100 μm²) fabricated in a trapezoidal microchannel (600 μm in width and 150 μm in depth). Experiments are carried out for six different surfactant concentrations of Triton X-100 ranging from 47 ppm to 2103 ppm, for mass flux in the range from 45 kg/m² s to 225 kg/m² s, pulse width in the range from 50 μs to 2 ms, and heat flux in the range from 3 MW/m² to 65 MW/m². As in existing work on pool boiling under steady heating, it is found that nucleate boiling becomes more vigorous and heat transfer is enhanced greatly with the addition of surfactant with maximum boiling heat transfer occurs at the critical micelle concentration (cmc). Furthermore, these maximum values of boiling heat transfer coefficient increase with decreasing pulse width. When concentration is below cmc, the heat flux needed for nucleation increases with increasing concentration and the nucleation temperature is reduced. When concentration is higher than cmc, the boiling heat transfer coefficient decreases and nucleation temperature is higher than that of pure water.     

Experimental investigation of bubble behaviors in subcooled flow boiling

The experimental investigation on vapor bubble growth is performed for analyzing subcooled boiling in a vertical annular channel with inner heating surface and upward water flow under atmospheric pressure. Bulk liquid mass flux ranges from 79 kg/m2s to 316 kg/m2s, and subcooling is from 40 K to 60 K. The bubble behaviors from inception to collapse are captured by High-speed photography. The performance of bubble growth recorded by the high-speed photography is given in this paper. The bubble behaviors, effect of the bubble slippage on the heat transfer, and various forces acting on the bubble are discussed.     

An Investigation on Heat Transfer to Supercritical Water in Inclined Upward Smooth Tubes

Within the range of pressures from 23 to 30 MPa, mass velocities from 600 to 1200 kg/(m²s), and heat fluxes from 200 to 600 kW/m², experiments have been performed for an investigation on heat transfer to supercritical water in inclined upward smooth tubes with an inner diameter of 26 mm and an inclined angle of 20° from the horizon. The results indicated that heat transfer characteristics of supercritical water are not uniform along the circumference of the inclined tube. An increase in the mass velocity of the working fluid can decrease and even eliminate the non-uniformity. Properties of supercritical fluid acutely vary with the temperature near the pseudocritical point. While the ratio of the mass velocity to the heat flux exceeded 2.16 kg/(kWs), heat transfer enhancement occurred near the pseudocritical point; conversely, heat transfer deterioration occurred while the ratio of the mass velocity to the heat flux was lower than 2.16 kg/(kWs). As the pressure increased far from the critical pressure, the amount of deterioration decreased. Correlations of heat transfer coefficients of the forced-convection heat transfer on the top and bottom of the tube have been provided, and can be used to predict heat transfer coefficient of spirally water wall in supercritical boilers.     

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.     

Study on onset of nucleate boiling in bilaterally heated narrow annuli

Onset of nucleate boiling (ONB) experiments using deionized water as working fluid have been conducted in a range of pressure from 1 to 4 MPa, mass flow velocity from 56 to 145 kg/m² s and wall heat flux from 9 to 58 kW/m² for vertical narrow annuli with annular gap sizes of 0.95, 1.5 and 2 mm. We found that the ONB sometimes occurs only on outer annulus surface, sometimes occurs only on inner annulus surface and sometimes occurs on both annulus surfaces. The heat flux of the other side has great influence on the heat flux of the ONB and the latter will decrease with the increase of the heat flux of the other side. It is also found that the heat flux of the ONB increases with the increase of the pressure, the mass flux and wall superheat. However, the heat flux of the ONB will decrease as the gap size increases in narrow annuli. The heat flux of the ONB in narrow annuli is much lower than that calculated by correlations for conventional channels and a new correlation, which has good agreement with the experimental data, has been developed for predicting the heat flux of the ONB in narrow annuli.