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1.
The thermal efficiency of a reheating furnace was predicted by considering radiative heat transfer to the slabs and the furnace wall. The entire furnace was divided into fourteen sub-zones, and each sub-zone was assumed to be homogeneous in temperature distribution with one medium temperature and wall temperature, which were computed on the basis of the overall heat balance for all of the sub-zones. The thermal energy inflow, thermal energy outflow, heat generation by fuel combustion, heat loss by the skid system, and heat loss by radiation through the boundary of each sub-zone were considered to give the two temperatures of each sub-zone. The radiative heat transfer was solved by the FVM radiation method, and a blocked-off procedure was applied to the treatment of the slabs. The temperature field of a slab was calculated by solving the transient heat conduction equation with the boundary condition of impinging radiation heat flux from the hot combustion gas and furnace wall. Additionally, the slab heating characteristics and thermal behavior of the furnace were analyzed for various fuel feed conditions. 相似文献
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Thermal analysis of a flat-plate boiling collector having sub-cooled inlet and saturated exit states
The analysis of the thermal performance of a boiling flat-plate solar collector is presented. A generalized heat removal factor and a new formulation for the overall thermal loss coefficient are developed. It is demonstrated that the conventional heat removal factor for non-boiling collectors is a limiting case of a more generalized result. The new formulation for the overall thermal loss coefficient is shown to be a function of the fractional non-boiling length of the flow channel. The influence of the inlet sub-cooling is evaluated and the operating limits of solar flat-plate collectors are determined. A comparison is made between the thermal model for boiling collectors having sub-cooled inlet states and experimental results. Favorable agreement is obtained. 相似文献
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A way of calculating the overall equivalent heat transfer coefficient of wheel rims of large capacity steam turbines is presented. The method and formula to calculate the mean forced convection heat-transfer coefficient of the surface of the blade and for the bottom wall of the blade passage, are introduced. The heat transmission from the blade to the rim was simplified by analogy to heat transmission in the fins. A fin heat transfer model was then used to calculate the equivalent heat transfer coefficient of the blade passage. The overall equivalent heat transfer coefficient of the wheel rim was then calculated using a cylindrical surface model. A practical calculation example was presented. The proposed method helps determine the heat transfer boundary conditions in finite element analyses of temperature and thermal stress fields of steam turbine rotors. 相似文献
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Jinyuan Shi Zhicheng Deng Yu Yang Ganwen Jun 《Frontiers of Energy and Power Engineering in China》2008,2(1):20-24
A way of calculating the overall equivalent heat transfer coefficient of wheel rims of large capacity steam turbines is presented.
The method and formula to calculate the mean forced convection heat-transfer coefficient of the surface of the blade and for
the bottom wall of the blade passage, are introduced. The heat transmission from the blade to the rim was simplified by analogy
to heat transmission in the fins. A fin heat transfer model was then used to calculate the equivalent heat transfer coefficient
of the blade passage. The overall equivalent heat transfer coefficient of the wheel rim was then calculated using a cylindrical
surface model. A practical calculation example was presented. The proposed method helps determine the heat transfer boundary
conditions in finite element analyses of temperature and thermal stress fields of steam turbine rotors.
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Translated from Journal of Power Engineering, 2007, 27(2): 153–156 [틫自: 뚯솦릤돌] 相似文献
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《International Journal of Heat and Mass Transfer》2007,50(3-4):452-463
Experiments were performed to explore the micro-channel cooling benefits of water-based nanofluids containing small concentrations of Al2O3. The high thermal conductivity of nanoparticles is shown to enhance the single-phase heat transfer coefficient, especially for laminar flow. Higher heat transfer coefficients were achieved mostly in the entrance region of micro-channels. However, the enhancement was weaker in the fully developed region, proving that nanoparticles have an appreciable effect on thermal boundary layer development. Higher concentrations also produced greater sensitivity to heat flux. Despite this enhancement, the overall cooling effectiveness of nanoparticles was quite miniscule because of the large axial temperature rise associated with the decreased specific heat for the nanofluid compared to the base fluid. For two-phase cooling, nanoparticles caused catastrophic failure by depositing into large clusters near the channel exit due to localized evaporation once boiling commenced. These and other practical disadvantages bring into question the overall merit of using nanofluids in micro-channel heat sinks. 相似文献
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In this study, the two‐phase heat‐transfer coefficient of R404A inside horizontal tubes is analyzed through the evaporator's overall heat‐transfer coefficient, obtained using the effectiveness—Number of Transfer Units thermal design approach. This method constitutes an approximation that can be used in the evaporator's thermal design with an attempt to break some of the initial assumptions established in the heat exchanger thermal design method development. For the analysis, an experimental refrigeration system that is commercially available is built up with a shell and tube evaporator. All the experiments are performed at different evaporator pressures (270, 570 kPa), evaporator temperatures (?20, 0°C) and cooling water temperatures (20, 40°C). For these parameters, overall heat‐transfer coefficient of the heat exchanger is found in the range of 0.05–0.35 kW °C?1. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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Jianhua Wang Zhixin Feng Qian Zhang Xiangyu Wu Shiyan Ma 《International Journal of Heat and Mass Transfer》2012,55(17-18):4800-4807
An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Rem (Reynolds number of hot mainstream) and Rec (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined. 相似文献
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日光温室加内保温幕的试验 总被引:4,自引:0,他引:4
减少夜间通过前屋面的散热是日光温室节能的关键。文章对室内前屋面下设置保温幕的试验温室(有幕温室)与相同结构的不设保温幕的对比温室(无幕温室)的温度环境进行了测试比较。测试结果显示:内保温幕的设置有效地抑制了夜间通过前屋面由于温差引起的散热,并提高了晴天夜间室内的温度。 相似文献
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The dimensioning, a thermal parameters analysis and the experimental heat loss coefficients of an adsorptive solar refrigerator prototype used for ice production are presented. The solar icemaker operates in an intermittent cycle, i.e. without recovering heat. It uses the activated carbon–methanol pair whose basic components are an adsorber coupled to a static solar collector, a condenser and an evaporator. Some innovations were considered, especially those brought about by French researchers, in which the adsorber was always box-shaped with extended surfaces, and air condensers were used. For the present system, the adsorber is bi-facially irradiated and covered with transparent insulation material (TIM), the geometric configuration of the main components is multi-tubular, and a water condenser is used. TIM polycarbonate covers are used on the top and bottom of the adsorber. The components of the prototype were dimensioned after the results from numerical simulations using meteorological data valid for the hottest six months in João Pessoa (7°8′S, 34°50′WG), whose climate is typically hot and humid. The machine was designed to produce up to 10 kg of ice/day per square meter of solar collection surface. Analyses of the thermal parameters influence on the ice production as well as parameters for dimensioning each component of the machine are presented. The overall heat loss coefficient by the top and the bottom of the adsorber–solar collector component are experimentally evaluated. The tests were performed using an incandescent lamp panel disposed on a 1 m2 surface, totalizing a thermal power of 3600 W. The results show a good efficiency of the TIM covers, achieving overall heat loss coefficient values between 0.54 and 1.90 W m−2 K−1. 相似文献
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Jorge A.W. Gut 《International Journal of Heat and Mass Transfer》2003,46(14):2571-2585
A mathematical model is developed in algorithmic form for the steady-state simulation of gasketed plate heat exchangers with generalized configurations. The configuration is defined by the number of channels, number of passes at each side, fluid locations, feed connection locations and type of channel-flow. The main purposes of this model are to study the configuration influence on the exchanger performance and to further develop a method for configuration optimization. The main simulation results are: temperature profiles in all channels, thermal effectiveness, distribution of the overall heat transfer coefficient and pressure drops. Moreover, the assumption of constant overall heat transfer coefficient is analyzed. 相似文献
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Heat exchangers have been widely used for efficient heat transfer from one medium to another. Nanofluids are potential coolants, which can afford excellent thermal performance in heat exchangers. This study examined the effects of water and CuO/water nanofluids (as coolants) on heat transfer coefficient, heat transfer rate, frictional loss, pressure drop, pumping power and exergy destruction in the corrugated plate heat exchanger. The heat transfer coefficient of CuO/water nanofluids increased about 18.50 to 27.20% with the enhancement of nanoparticles volume concentration from 0.50 to 1.50% compared to water. Moreover, improvement in heat transfer rate was observed for nanofluids. On the other hand, exergy loss was reduced by 24% employing nanofluids as a heat transfer medium with comparing to conventional fluid. Besides, 34% higher exergetic heat transfer effectiveness was found for 1.5 vol.% of nanoparticles. It has a small penalty in the pumping power. Hence, the plate heat exchanger performance can be improved by adapting the working fluid with CuO/water nanofluids. 相似文献
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An insulated north wall greenhouse dryer has been fabricated and tested for no-load condition under passive mode. Testing has been conducted in two different cases. Case-I is considered for solar collector kept inside the dryer and Case-II is dryer without solar collector. Convective heat transfer coefficient and various heat transfer dimensionless numbers with have been calculated for thermal analysis. The maximum convective heat transfer coefficient is found 52.18?W/m2°C at 14?h during the first day for Case-I. The difference of the highest convective heat transfer coefficient of both cases was 8.34?W/m2°C. Net heat gain inside room curves are uniform and smooth for Case-I, which shows the steady heat generation process due to presence of solar collector inside the dryer. Above results depicts the effectiveness of solar collector and insulated north wall. The selection of suitable crop for drying can be done by analysing article’s result. 相似文献
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R.K. Mazumder N.C. Bhowmik M. Hussain M.S. Huq 《Energy Conversion and Management》1986,26(3-4):313-316
Tubular receivers with an evacuated space between the absorber and concentric glass cover to suppress convection heat loss are employed as absorbers of linear concentrators in the intermediate temperature range. A knowledge of their heat loss factor is important for a study of the thermal performance of such solar concentrating systems. The heat loss factor of a collector can be calculated by solving the governing heat transfer equations or estimated from an empirical equation, if available. The governing equations must be solved simultaneously by iterations, but this is tedious and cumbersome. Although several correlations exist for determining the heat loss factor for flat-plate collectors and non-evacuated tubular absorbers of linear solar collectors, there is no available correlation for predicting the heat loss factor of evacuated receivers.
A correlation to calculate the heat loss factor (UL) of evacuated tubular receivers as a function of variables involved (absorber temperature, emittance, diameter and wind loss coefficient) has been obtained. The correlation developed by a least square regression analysis predicts the heat loss factor to within ±1.5% of the value obtained by exact solution of the simultaneous equations in the following range of variables: wind loss coefficient, 10–60 W/m2°C; emittance, 0.1–0.95; and absorber temperature, 50–200°C. 相似文献
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热管式真空管集热器的热性能研究 总被引:34,自引:9,他引:34
在传热分析的基础上,提出了确定热管式真空集热器的总热损系数、效率因子和热转移因子的计算方法,通过实验,还分别测定了单根真空管和真空管集热器的瞬时效率曲线。实验结果与理论计算值符合较好。 相似文献
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《International Communications in Heat and Mass Transfer》1995,22(5):671-681
This work brings out the effect of entry length on the mean overall heat transfer coefficient of a heat exchanger for turbulent flow with low Prandtl Number. For this purpose simple heat transfer correlations are proposed in the thermal entry length region and with these correlations the correction factors are calculated for selected special cases. The effect of flow parameters and total heat exchanger length has also been investigated. Finally comments are made regarding thermal boundary conditions and in its light the commonly used design method is assessed. 相似文献
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Abstract Accurate data have been obtained on the thermal conductivity of six grades of manufactured graphite. In particular, the effect of orientation, manufacturing process, and impregnant content on the thermal conductivity was investigated. Using the thermal conductivity data, a three-dimensional finite-element model has been set up to determine the wall resistance of a particular graphite block. The effect of the anisotropy of the extruded graphite on the wall resistance was investigated. It was found that the anisotropy can be neglected if the wall resistance is calculated by assuming that the blocks have an isotropic thermal conductivity equal to the thermal conductivity perpendicular to the direction of extrusion. Also, the phenol formaldehyde film on the surface of the heat exchanger channels was determined to have a significant thermal resistance. It was calculated that, for typical operating film coefficients with water on both sides, the overall heat transfer coefficient can be expected to increase by more than 50% if the resin layer is removed. 相似文献
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