斯特林发动机管式加热器内振荡流动的换热特性实验研究

模拟管式加热器在斯特林发动机中的工作状态,研究加热器管内工作气体振荡流动的换热特性,得到气体压力、振荡频率等对换热的影响规律,进一步得到工作气体与管壁间的平均换热系数,并将结果转化为无量纲参数,比较稳定流动换热关联式计算结果与实验结果的偏差。实验结果可对斯特林发动机管式加热器设计、优化和换热性能预测提供参考。     

Study on overall heat transfer coefficient for a rotating cavity type heat exchanger

In this study, overall heat transfer coefficient for a rotating cavity type heat exchanger has been investigated. The heat exchanger mainly consisted of a rotating cylindrical cavity with axially throughflow. The cylindrical cavity was rotated in a stationary housing. An axial throughflow of cold water was supplied in the cavity through a pipe rotating with cavity. The cold water left the cavity via an identical pipe. Hot water flowed between rotating cylindrical cavity and stationary housing Experiments were conducted to obtain the effects of cold water flow rate, hot water flow rate, and rotational speed on overall heat transfer coefficient.     

有限热源斯特林热机的优化性能研究

应用有限时间热力学方法,探索有限热源、热阻和回热损失的斯特林热机的优化性能,得到一些新的性能参数,所得结论可为斯特林热机的研制和优化设计提供些新理论指导。     

Numerical study on heat transfer of Stirling engine heater tube with rectangular micro-ribs

Abstract

Numerical simulations were carried out to study the heat transfer and friction characteristics for Stirling engine heater tubes with three-dimensional internal extended micro-rib. During the numerical simulations, phase angle in a cycle ranged from 0° to 360°. The results represent that the friction factor (f) increased with micro-rib length (L) and micro-rib height (H) for enhanced tube within the range of parameters in this study. For the micro-rib heat tube of L?=?60 mm and H?=?0.9 mm, the j was equal to 1.44 and the average Stanton number St was significantly improved up to 1.287 compared to smooth tube, the friction factor f was also increased by almost 2.32. The outlet pressure from 0.7 Mpa to 1 Mpa with the cosine change is presented as the phase angle increases. Therefore the benefits are apparent for micro-rib heat tube in Stirling engine.     

The effect of heat transfer on the pressure drop through a heat exchanger for aero engine applications

This work is focused on the experimental study of the performance of a heat exchanger designed for aero engine applications. The heat exchanger is operating as a heat recuperator by taking advantage of the thermal energy of the exhaust gas of the aero engine in order to obtain a better combustion with less pollutant emissions. The experimental study has been performed in a wind-tunnel by taking detailed flow and thermal measurements on a 1:1 model of the heat exchanger under various operating conditions described by the hot gas inlet mass flow rates and its spatial direction (different angles of attack and inclination) towards the heat exchanger. The hot gas has been modeled with preheated air. Six sets of measurements have been carried-out for different hot gas inlet and outlet temperatures, including also isothermal measurements without any heat transfer in order to have a reference point for the pressure drop of the flow through the device. The experimental results showed that the effect of the angle of attack on the pressure drop is significant while the effect of the angle of inclination is negligible. Additionally, the pressure drop through the heat exchanger is greatly affected by the heat transfer.     

The effect of area ratio on microjet array heat transfer

The heat transfer performance of five submerged and confined microjet arrays using air and deionized water as the working fluids was investigated. Both inline and staggered array arrangements of jet with diameters of 54 and 112 μm were investigated, and the area ratio (total area of the jets divided by the surface area) was varied between 0.036 and 0.35. Reynolds numbers defined by the jet diameter were in the range of 180–5100 for air and 50–3500 for water. A heat flux of 1100 W/cm² was obtained at a fluid inlet-to-surface temperature difference of less than 30 °C. The results were compared with established correlations, and no evidence was found to suggest that the behavior of submerged and confined jets at the microscale is fundamentally different than at the macroscale. Reynolds number, Prandtl number, and area ratio were found to significantly affect the heat transfer performance, and a curve fit was developed, which correlated 290 of the 295 data points within ±25% with an MAE of 11%.     

The effect of displacer material on the performance of a low temperature differential Stirling engine

In this study, a gamma‐type low temperature differential Stirling engine was designed and manufactured. The displacer and piston of the engine were concentrically situated to each other. The engine was tested by using a liquefied petroleum gas burner at laboratory conditions. The working fluid was ambient air at atmospheric pressure. Test procedure intended to investigate the speed‐torque and speed‐power characteristics of the engine depending on the hot‐end temperature. Two different displacers made of aluminum alloy and medium density fiberboard were used. The maximum torque and power obtained were 0.166 Nm at 125 rpm speed and 3.06 W at 215 rpm speed, respectively, at 160 °C hot‐end temperature with medium density fiberboard displacer. Copyright © 2011 John Wiley & Sons, Ltd.     

Heat conduction in the hollow sphere with a power-law variation of the external heat transfer coefficient

The conduction phenomenon in an insulated sphere is re-worked through a dimensionless approach, where the heat transfer coefficient dependence on the external radius and on the surface temperature, as in the case of forced and free convection, is taken into account. Assuming a power law variation of the convection coefficient [1, 2], and using the results of Sparrow [3], equations and graphs for the most important dimensionless parameters are presented. The developed equations show, for example, that as the insulation thickness increases the heat transfer rate tends to a positive value, independent of the considered case: constant convection coefficient, forced or free convection.     

Effects of surface roughness and tube materials on the filmwise condensation heat transfer coefficient at low heat transfer rates

The laminar filmwise condensation heat transfer coefficient on the horizontal tubes of copper and stainless steel was investigated. The outside diameter of the tubes was 15.88 mm, and the tube thickness ranged from 1.07 to 1.6 mm. The polished stainless steel tube had an RMS surface roughness of 0.37 μm, and commercial stainless steel tubes had maximum surface roughness of 15 μm. The tests were conducted at saturation temperatures of 20 and 30 °C, and liquid wall subcoolings from 0.4 to 2.1 °C. The measured condensation heat transfer coefficients were significantly lower than the predicted data by the Nusselt analysis when the ratio of the condensate liquid film thickness to the surface roughness, δ / R_p–v, was relatively low. When the condensate liquid film was very thin, tube material affected the condensation heat transfer coefficient in the filmwise condensation.     

Robust methodology for determination of heat transfer coefficient distribution in convection

To predict the microstructures, residual stresses and distortions in the heat treated metal components, it is important to accurately know the heat transfer coefficients (HTCs) between the hot work piece and cooling media. In this paper, a new method is presented to accurately determine the node-based HTC distribution by coupling computational fluid dynamics (CFD) with optimal weight functions and scale factors. With this new method, the predicted temperature profile of the work piece during quenching (rapid cooling) is in excellent agreement with experimental measurements. This new method can be also applied to accurately predict convection heat transfer in thermal equipment such as heat exchangers and refrigerators, building thermal design and other heat transfer related situations.     

花岗岩岩芯单裂缝水流总传热系数的实验研究

为提高在流动水与花岗岩断口壁之间传热特性的认识进行了实验和理论研究。在0、3、6 MPa的围压和70、80、90、100℃的温度下,对特征良好的花岗岩试样中单裂缝的流动和传热进行了系统的实验研究。根据实验数据对现有四种总传热系数(OHTC)公式进行了评价。相比于其他可能预测出不正常OHTC值的公式,D公式和新提出的E公式在我们的实验中拟合较好。提出的新方法理论基础严整、形式简洁并且能够有效地预测结果,值得推荐使用。根据提出的公式E,发现OHTCs与流量呈正相关。然而,随着孔隙的增宽,OHTCs会变小,这意味着传热效率也会随着下降。此外,大多数OHTC-孔径曲线的斜率是相似的。最后研究了OHTC公式可能产生异常值的条件。     

The impact of heat exchanger fouling on the optimum operation and maintenance of the Stirling engine

This paper focuses on the effect of heat exchanger fouling on the performance of the Stirling engine in combined heat and power (CHP) application. Fouling results from using biomass fuels and affects the heat exchanger that transfers heat into the engine. This heat exchanger is referred to as the heater. The heat exchanger that recovers heat from the flue gases is also affected by fouling. To determine the performance of the Stirling engine, a commercial Stirling analysis tool is applied together with models that have been developed for the heat transfer in the heater, regenerator and cooler of the engine. The Stirling engine model uses constant temperatures for the heat addition and rejection, with the theory of displacement engine as a basis. The fouling in the heat exchanger is taken into account by using a fouling factor that corresponds with the degradation in the total heat transfer coefficient. The Stirling engine model together with the model for heat exchanger fouling makes it possible to estimate the effect of fouling on the performance of the Stirling engine. A cost model is developed for the engine to translate changes in performance into economy in CHP operation. In the studied application, the Stirling engine is operated by the heat demand. Together with the selected control method, performance and cost models compose a tool for the simulation and optimization of the system. The use of the models to determine the optimal cleaning interval of the heat exchanger surfaces is considered.     

考虑污垢因素的凝汽器传热系数的计算研究

凝汽器冷却水系统运行过程中,由于难溶盐分的沉积和微生物污垢的生成,会在凝汽器的换热表面上形成污垢,使传热系数下降,真空降低。分析了凝汽器水侧污垢的特性,然后给出了考虑污垢因素的凝汽器传热系数的计算方法。     

The investigation of the effect of thermal barrier coating on the performance of Stirling engine

The shuttle heat transfer is one of the reasons reducing the performance of Stirling engines. This study is concerned with the reduction in shuttle heat transfer by coating the displacer. The displacer of a gamma type Stirling engine was coated with a layer of yttria‐stabilized zirconia (YSZ), and the effect of the coating on the engine performance was evaluated by comparing speed‐power and speed‐torque characteristics of the engine with coated and uncoated displacers. Characteristics were obtained for 700, 800 and 900°C heater temperatures. At each stage of the heater temperature, the charge pressure ranged from 1 to 3.5 bars with 0.5 bar increments. At 900°C heater temperature and 3 bars charge pressure, the shaft power before coating was 34.9 W, after coating the power increased to 43.8 W, which corresponds to a 25% increment. The temperature applied to the engine did not cause any damage on the coating layer. Copyright © 2008 John Wiley & Sons, Ltd.     

北方地区岩土导热系数及换热量的测试研究

岩土导热系数是地源热泵地埋管换热器的重要设计参数;测井单位深度换热量是地埋管换热器系统的设计依据。掌握工程区域岩土的热物性及换热性能,是保证地源热泵系统高效、稳定运行的关键。文章建立了现场测试岩土导热系数及换热量的方法,并结合沈阳浑南高新技术产业开发区某地源热泵工程,测试分析了岩土导热系数和测井单位深度换热量。结果表明,该区域的岩土具有较好的导热能力,适合采用地埋管地源热泵系统;在特殊地理条件下设计地源热泵系统方案前,应对拟建区域的地质条件进行全面勘探,以优选工程区域,为岩土热响应测试结果的可靠性提供保障。     

Effect of heat recovery on the performance of a shape-memory effect heat engine

The performance of a shape-memory effect heat engine employing heat recovery has been modelled taking into account heat transfer to the working element and non-ideal material behaviour. Calculations of the energy flux to and from the working element show that a substantial fraction of the sensible heating requirements can be recovered by heat transfer between the heating and cooling stages. The increase in efficiency due to heat recovery and the resulting power output are shown to be a function of stress and rate of temperature change of the heat-transfer fluid. Working element-fluid heat transfer and hysteresis behaviour of the shape-memory working element are found to limit the efficiency improvement associated with heat recovery.     

The effect of gap distance on the heat transfer between a heated finned surface and a saturated porous plate

Experiments were performed to investigate the effect that the presence of a gap has on the heat transfer between a heated finned surface and a saturated porous plate with an average pore radius of 200 μm. There was evidence that the vapour generated beneath the heated surface can escape to the vapour grooves more easily when a gap distance is introduced. This seemed to decrease the vapour penetration into the porous plate. The heat transfer performance of the heated finned surface initially increased as the gap distance was increased from 0 to 500 μm, but remained relatively unchanged for gap distances of 500–900 μm.