对流型地层内井下换热器实验模拟研究

采用3-5mm的饱和微珠玻璃球堆来模拟同轴管式井下换热器（DCHE）周围对流型地温地层，对其换热过程中温度分布及其变化、主要影响因素进行了实验研究，为利用同轴管式井下换热器开发地热资源提供依据。     

地热井下换热器传热性能的实验模拟研究

对基于井下换热器的模拟装置进行实验研究.井下换热器为U型铜管,采用装有35mm玻璃珠的方形箱体模拟渗透性含水层.通过改变U型管和井筒的几何参数实现不同尺寸的井、管配合,研究了U型管换热器入口水温、流量、含水层渗流速度等参数对井下换热器热输出的影响,得到了包括以上因素的井下换热器传热实验关联式.结果表明:与渗流速度相比,含水层热储与U管入口之间的温差是影响热输出的主要因素;热输出总量与该温差并非为线性关系,两支管外侧的平均对流换热系数在中等温差下达到最大值,而增加U型管内水流速可进一步提高外侧表面平均传热系数.     

多孔介质层内同轴管式井下换热器的相似解

对多孔介质层内同轴管式井下换热器的自然对流传热过程进行了数值模拟。通过对井下换热器分段并依次在局部采用对流相似解法对其周围遵循Ｄａｒｃｙ定律的多孔介质层内自然对流过程进行了近似求解，得到了有关多孔介质层物性参数对井下换热器热输出影响的规律。     

单U形管型井下换热器的热量输出特性（Heat Extraction Characteristics of a Suigle U-Tube Downhole Heat Exchanger,田子真、盛田耕二、营原片洋、藤田忠和马渡慎吾，日本地热学会志，Vol．26，No．2（2004），107-120）

为了从浅层地热资源直接取出热量，已经提出了若干种井下换热器，如：热虹吸型热管、同心管热虹吸、井下同轴换热器和U形管井下换热器。其中，U形管井下换热器因为费用低和容易制造，在日本以及全世界都被广泛使用。目前，单U形管井下换热器的热量输出特性已经作了数值模拟研究。     

错列翅片紧凑式换热器湍流流动及换热性能的数值研究

采用高雷诺数κ-ε湍流模型，对中高雷诺数下紧凑式错列翅片换热器的表面换热及流动特性进行数值模拟。结果表明，该种型式的换热器具有良好的流动和换热性能，拓宽了其空调领域的应用。     

水-油连续螺旋折流板换热器热力性能研究

利用计算流体力学(Computational Fluid Dynamic, CFD)方法，针对连续螺旋折流板换热器建立物理模型和数学模型，在管侧介质为水和壳侧介质为原油条件下，研究不同原油流量及螺旋角对螺旋折流板换热器内部流场、换热性能及阻力性能的影响，并拟合了水油换热时螺旋折流板换热器的Nu、f与Re的关联式。结果表明：22°螺旋角的螺旋折流板换热器与其它较小螺旋角换热器对比，壳侧压降和换热系数逐渐减小，综合换热性能最佳。通过对壳侧原油为层流状态下的阻力系数和对流换热系数关系式进行拟合，更好地指导水-油连续螺旋折流板换热器的热力设计。     

圆柱阵列换热器底部壁面对流换热的初步研究

圆柱阵列换热器是最具潜力的太阳能电站换热器。该文分析了圆柱阵列换热器底部壁面对流换热特性。编制了基于最小二乘法的计算程序，并据此利用现有实验数据拟合出具有普遍意义的圆柱阵列换热器底部壁面对流换热的无量纲换热参数的表达式，对比分析表明，该表达式真实地反映了圆柱阵列的存在及其尺度的变化对底部壁面换热的影响。这一结果为优化换热器结构和建立研究太阳能换热器的多区域连续数值模型奠定了基础。     

地热井下换热器研究与应用进展

介绍了地热井下换热器的国内外研究进展和应用现状,着重阐述了地热井下换热器研究与设计的难点问题,包括地热井下换热器回水管散热损失、井管直径的影响、对流增速管的作用以及进水温度对总提取热量的影响.指出井下换热器与地面热泵系统的结合,降低了井下换热器的进水温度,从而可以有效地提高从地下提取的总热量,而且可以降低对地下热储层温度上限的要求.另外,分析了在我国应用地热井下换热器的可行性与主要使用范围.     

沉浸式污水换热器的传热模型与验证

沉浸式污水源热泵系统可避免机组或换热器的腐蚀,便于换热器的清洗,已在工程中应用.文章建立了沉浸式污水换热器的传热模型,并通过实验验证了模型的准确性;在污水流量变化的情况下,分别测试了沉浸式换热器在冬、夏季的传热系数.实测结果表明,采用高密度聚乙烯管的沉浸式污水换热器单位长度的传热量约为100 W/m.     

导热型地温梯度场内的U型管式井下换热器

建立了一个描述“Ｕ”型管式地热井下换热器在导热型地温梯度内热性能的数学模型。该模型考虑了地下岩层的温度分布及岩层与其下面对流层的对流换热对“Ｕ”型管式地热井下换热器热性能的影响，并对井下换热器流体进口参数的变化，岩层热物性及温度剖面变化对热输出的影响进行了计算。     

Turbulent direct-contact heat transfer between two immiscible fluids

Heat transfer between two immiscible liquid phases in turbulent flow is of great interest in improving the residence time, compactness, and energy cost of cooling and heating processes. The high-efficiency vortex (HEV) device used here as a direct-contact heat exchanger (DCHE) is a generic multifunctional exchanger/reactor in which wall tabs generate longitudinal vortices responsible for convective radial transfer that enhance macro-mixing, phase dispersion and fast temperature homogenization in the flow.The experiments reported here concern a continuous flow of water in which an immiscible mineral oil is injected. The inlet water temperature ranges from 11 to 13 °C, and the inlet oil temperature from 40 to 48 °C; the flow Reynolds number varies between 7500 and 15 000. An algebraic one-dimensional thermal model accounting for the axial evolution of the phase temperatures coupled with drop breakup is developed and validated by the experimental thermal results in the DCHE. This model requires knowledge of the turbulent field in single-phase conditions; it can be adapted to other flow geometries and can be used as a sizing tool for engineering design.Despite the phase separation at the outlet, the DCHE is more efficient than a double-jacketed heat exchanger in terms of global Nusselt number. In addition, the HEV heat exchanger is energetically less costly than the other DCHE for the same heat-transfer capacity.     

蒸发冷却板式换热器的传热与阻力实验研究

通过对板式换热器传热的理论分析和在干工况、喷淋工况下的实验研究,得出板式换热器在喷淋情形下空气侧喷淋水量对强化对流传热系数的影响关系,进而拟合出喷淋情形下空气侧的对流传热系数的关联式。同时为了综合考虑由于喷淋造成的换热器性能的变化,还对喷淋前后空气侧的阻力变化进行了测试分析。     

Heat extraction characteristics by coaxial heat exchanger

This study was performed to investigate the heat extraction characteristics from shallow geothermal resources using a coaxial heat exchanger. First, a computer simulation program for a coaxial heat exchanger was checked and verified by laboratory experiments. After inspecting the effectiveness of the computer program described herein, a numerical simulation for a real scale model was conducted under the condition that the heat transfer mechanism in the stratum was heat conduction. Unsteady heat extraction characteristics are presented herein, and the effects of the tube material, inner diameter, and circular modes on the heat extraction rate are discussed. From the computer results it was found that the heat extraction performance using a coaxial heat exchanger greatly depended on the factors mentioned above in the range of the parameters covered in this study. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 496–513, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20083     

Design and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS)

The present study considers the design, performance analysis and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS). The optimum mass flow rate of the geothermal fluid for minimum pumping power and maximum extracted heat energy was determined. In addition, the coaxial pipes of the downhole heat exchanger were sized based on the optimum geothermal mass flow rate and steady-state operation. Transient effect or time-dependent cooling of the Earth underground, and the optimum amount and size of perforations at the inner pipe entrance region to regulate the flow of the geothermal fluid were disregarded to simplify the analysis. The paper consists of an analytical and numerical thermodynamic optimization of a downhole coaxial heat exchanger used to extract the maximum possible energy from the Earth's deep underground (2 km and deeper below the surface) for direct usage, and subject to a nearly linear increase in geothermal gradient with depth. The thermodynamic optimization process and entropy generation minimization (EGM) analysis were performed to minimize heat transfer and fluid friction irreversibilities. An optimum diameter ratio of the coaxial pipes for minimum pressure drop in both limits of the fully turbulent and laminar fully-developed flow regime was determined and observed to be nearly the same irrespective of the flow regime. Furthermore, an optimum geothermal mass flow rate and an optimum geometry of the downhole coaxial heat exchanger were determined for maximum net power output. Conducting an energetic and exergetic analysis to evaluate the performance of binary power cycle, higher Earth's temperature gradient and lower geofluid rejection temperatures were observed to yield maximum first- and second-law efficiencies.     

Transient numerical model for a coaxial borehole heat exchanger with the effect of borehole heat capacity

This paper proposed a transient numerical model for a coaxial borehole heat exchanger, which considered the impact of borehole specific heat capacity. The fluid vertical temperature distribution inside the coaxial borehole heat exchanger (BHE) had been predicted based on MATLAB and compared with other transient models. Validated by measured data from a thermal response test, the built model agreed better than other models, especially in short times, with a relative error of 3.63% in 2 hours. Then, the quantitative influences of borehole specific heat capacity and other parameters on thermal performance of borehole heat exchangers were specified.     

Transient heat flux measurement analysis from coaxial thermocouples at convective based step heat load

The measurement of surface heating rate is an imperative parameter in the force convection ground-based facility for short duration investigation due to the heat transfer rate is changing rapidly. The coaxial thermocouples are suitable to measure the transient heat flux in fast varying heat transfer application because of having fast response time in the range microseconds or less. In this addition, the K-type, E-type, and J-type of coaxial thermocouples are contrived as well as the thermal coefficient resistance (TCR) and sensitivity (S) has been calculated from oil-bath based technique. These handmade coaxial thermocouples are exposed in a forced convection flow facility having three different input step heat loads as well as their transient heat fluxes are estimated using one-dimensional heat conduction modeling for the semi-infinite body. The numerical simulation has also been carried out with the analogous experimental parameters using ANSYS-FLUENT v.15.0 and compared with the outcome of the experimental approach and it is found that the average value of the transient temperatures having 0.3% error and surface heat flux recovered from this temperature is 10%. This study reveals the measuring ability of these handmade coaxial thermocouples at low temperature and low velocity on short duration transient measurements.     

Effect of inclination angle on free convection thermal performance of louver finned heat exchanger

The effect of inclination angle on the louver finned tube heat exchanger subject to natural convection condition is reported in this study. It is found that the inclination angle plays an importance role on the performance of the louver finned heat exchanger. Performance of the heat exchanger is associated with the interactions between fin, louver, tube, and inclination angle. The heat transfer performance generally decreases with the rise of the inclination angle. This decrease of heat transfer performance is due to the blockage fin and its reversed heat dissipating direction against the raising air. However, at an inclination angle such as 30–45°, a considerable increase of heat transfer performance is seen. This is because appreciable amount of air flow was directed by the louver, causing a “louver-directed” phenomenon as that of in forced convection. With a further increase of inclination angle, the blockage effect caused by the fin is so strong as to offset the “louver-directed” phenomenon. Unlike those shown in force convection, the heat transfer performance decreased with the number of tube row.     

ROUGH SIZE ESTIMATION OF A THERMALLY FRACTURED ZONE IN AN INFINITE HOT ROCK MASS

Abstract

Concerning the extraction of geothermal energy from a deep thermal reservoir by the downhole coaxial heat exchanger with a thermally insulated inner pipe proposed by Morita et al, we obtained rough estimates of a size of the fractured zone induced by thermal stresses due to injecting cold water into the hot rock mass through the pipe. We assumed complete spherical symmetry of the temperature and stress fields. At the rough estimation, we considered three typical or extreme cases. (1) The fracturing affects neither the loading capacity of a fractured rock mass nor the temperature distribution within the formation. (2) The fractured zone completely loses its loading capacity and is fully invaded by the borehole water. No disturbance of the fracturing makes any difference in the temperature. (3) The rock formation is assumed to have an appropriately increased fictitious conduction substituted for the heat transfer enhanced by the expected convection within the fractured zone in order to discuss the effects of an occurrence of heat convection within the fractured zone on the temperature and stress distributions and the fractured zone size. As a result, the size of the zone has been estimated to be about ten or more times the borehole radius.     

双管程热交换器换热特性研究

通过对双管程热交换器结构和工作原理介绍,利用经典对流换热理论,采用定性判断和定量比较的方法,分析双管程热交换器与管壳式换热器的换热能力。对双管程热交换器而言,换热能力上,竞争优势明显;流动阻力上,无明显优势。在低品位能源余热利用方面,对深化双管程热交换器的研究,优化设备结构、降低成本有一定意义,值得工程技术人员借鉴与参考。