北京平原地热资源与地热供暖

对北京平原区地热资源条件和地热供暖现状进行了概述、分析了地热供暖技术的特点、优势以及当前制约其推广发展的"瓶颈".针对北京平原地热资源的可持续发展提出了两点具体建议.     

Heat Transfer Enhancement and Vortex Flow Structure in the Spirally Fluted Tubes

The turbulence kinetic energy and heat transfer performance of air in spirally fluted tube were numerically studied at a constant wall temperature with Reynolds...     

余热锅炉通流结构的三维数值模拟及结构优化

利用CFD软件对某高炉煤气余热锅炉通流结构的流动情况进行模拟,针对结构内流动不均匀的现象,提出了添加导流管的设想,并分析了通流结构中加入不同个数导流管对流场、不均匀性系数和湍流强度的影响。根据导流管个数对不均匀性系数和湍流强度的影响结果,对通流结构进行优化设计。研究表明:加入导流管的通流结构的不均匀性系数和湍流强度均明显优于原始的结构,导流管个数对不均匀性系数和湍流强度的影响很大。加入导流管可以均匀拐角处流场,提高受热面的传热有效性,同时对锅炉通流结构优化具有一定的参考价值。     

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m²-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m². The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.     

百叶窗结构参数对管带式散热器流动传热特性影响的研究

本文利用CFD计算软件star ccm＋,建立了管带式散热器空气侧的三维热固耦合模型,分析了不同空气流速和百叶窗结构参数对散热器流动和传热特性的影响,发现阻力损失主要集中在空气进入百叶窗的入口区域,百叶窗的前端传热效果较好,同时计算区域后半部分存在滞止区域;随着百叶窗倾角的增大,传热j因子先增后减,摩擦f因子一直增加,24°时传热j因子达到最大值;随着百叶窗厚度的增大,传热效果变差,摩擦f因子先增后减;百叶窗间距增加到一定程度,传热j因子和摩擦f因子增加缓慢,甚至减小。     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.     

Air-Side Flow and Heat Transfer in Compact Heat Exchangers: A Discussion of Enhancement Mechanisms

Abstract

The behavior of air flows in complex Heat exchanger passages is reviewed with a focus on the heat transfer effects of boundary-layer development, turbulence, spanwise and streamwise vortices, and wake management. Each of these flow features is discussed for the plain, wavy, and interrupted passages found in contemporary compact Heat exchanger designs. Results from the literature are used to help explain the role of these mechanisms in Heat transfer enhancement strategies.     

Review on Heat and Fluid Flow in Micro Pin Fin Heat Sinks under Single-phase and Two-phase Flow Conditions

This article reviews recent studies on the hydrodynamic and thermal characteristics of micro pin fin heat sink (MPFHS). In the studies reviewed in this article, liquid coolants such as water, HFE-7000, HFE-7200, R-123 were tested under both single-phase and two-phase flow conditions. Analytical, computational and experimental research studies were covered with a focus on configurations with traditional arrangements of micro pin fins (MPF) as well as original designs such as oblique finned MPFs, variable density MPF, vortex generators and herringbone structures. Single-phase flow results highlighted pressure drop penalty with achieved heat transfer enhancement. Many studies revealed the inability of conventional correlations to predict the hydrodynamic and thermal characteristics and proposed new correlations for different operating conditions and geometrical specifications. Regarding the studies on two-phase flows the number of performed studies is less than the ones in single-phase flow regime although the diversity of utilized coolants is more. Under flow boiling conditions, the focus was on determining flow patterns among MPFs for different arrangements and under different operating conditions. Unlike the studies on single-phase flows, the data could be relatively well predicted using the earlier suggested model by Lockhart and Martinelli with appropriate coefficients for different arrangements of MPFs.     

Heat Transfer and Fluid Flow on Dimpled Surface With Bleed Flow

This study investigates the effects of bleed flow on heat transfer and fluid flow on a dimpled surface in a rectangular channel. The heat transfer on a dimpled surface with bleed flow is compared with that on a dimpled surface without bleed flow. The height of the channel is 15.0 mm. The dimples are arrayed in staggered on the bottom surface of the channel with a pitch of 15.0 mm. The dimple depth is 3.75 mm and the dimple footprint diameter is 13.0 mm. The bleed hole is installed on the inner surface of the dimple and the diameter of the hole is 1.3 mm. The tests were conducted with varying Reynolds numbers from 1000 to 10,000 and 0.5% of total mass flow is flowing out through a bleed hole. A numerical method was employed to determine the detailed heat transfer coefficients. Commercial computational fluid dynamics software, ANSYS CFX 13.0, is adopted and the Shear Stress Transport model is set to turbulent model. As a result, the overall heat transfer rate on dimpled surface with bleed flow is 10–20% higher than that without bleed flow.     

Heat Transfer and Flow Characteristics of Al2O3/Water Nanofluid in Various Heat Exchangers: Experiments on Counter Flow

Abstract

On account of nanofluids influence on heat exchangers (HEs), a vigorous discussion can be made to concurrently contrast HEs to one another under the same conditions to detect maximum efficacy. Based on an extensive experimental study, this research is established to examine the effect of nanofluids on the performance of heterogeneous HEs with the same heat transfer surface area considering counter flow arrangement. A double pipe HE, a shell and tube HE and a plate HE are intended to accomplish the experiments. The experiments are executed under turbulent flow conditions using distilled water and Al₂O₃/water nanofluid with 0.2, 0.5, and 1% particle volume concentrations. From the results shown in the article, the double pipe HE revealed the best outcome for the heat transfer coefficient with a maximum enhancement of 60% while a maximum enhancement in the heat transfer coefficient of 11% was reported for the plate HE. Utilizing a nanofluid represented the lowest penalty in the pressure drop with a maximum enhancement of 27% for the plate HE while the highest penalty in the pressure drop with a maximum enhancement of 85% was observed in the double pipe and shell and tube HEs.     

Numerical Simulation of Re-Entry Flow: Heat Flux Evaluation

Abstract

The problem of heat transfer in a four channel plate heat exchanger involving the effect of unsymmetrical heat transfer in the outer two channels is studied analytically and experimentally. An energy balance over a control volume yields the governing system of differential equations that has been solved exactly for the cases of parallel flow and counterflow to give the temperature distribution in the channels. The results show that zero or even reversed heat flow may be obtained at the middle plate of the heat exchanger. Expressions for the heat exchanger efficiency and the log-mean temperature difference correction factor in terms of the heat capacity rate ratio and the number of transfer units are presented. Experiments carried out with a counterflow plate heat exchanger show reasonable agreement between the experimentally observed and the theoretically calculated efficiencies and mean driving temperature differences.