江水源热泵机组年运行能耗预测方法研究

在建筑工程的可行性研究阶段,空调系统冷热源的年运行能耗预测是静态投资回收期分析、全寿命周期成本分析中的核心组成部分,是冷热源选型的重要依据之一.然而,在实际项目中通常选用IPLV/NPLV、设计工况COP进行年运行能耗估算,计算结果与实际能耗有较大的误差.在已知建筑设计冷、热负荷的前提下,引入平衡温度概念,采用BIN方法预测不同温度下的建筑负荷,通过参数修正的方式预测江水源热泵机组在部分负荷率情况下功率,将热泵机组功率修正系数的变化函数拟合为水源侧进水温度和负荷侧回水温度的多项式.对长江上游地区江水温度及气象参数进行监测,并建立了江水温度与空气于球温度的数学关系.提出江水源热泵机组年运行能耗预测方法.该方法反映了系统运行的实际特征,且计算过程较为简单,可以在实际工程中应用.     

生物质发酵升温提高沼气池的池温和保温技术

分析了北方地区冬季沼气池产气量偏低的原因和沼气池池温与气温的变化关系,根据生物质(农作物秸秆)发酵升温的原理,利用麦秸发酵产热对提高沼气池池温进行了的试验.结果表明:生物质发酵升温能提高沼气池池温和产气量,解决沼气池在冬季不能正常使用的问题.文章对在沼气池拱顶上部改建生物质发酵升温池的技术工艺进行了介绍.     

A theoretical model for the prediction of the critical heat flux in heated microchannels

A theoretical model for the prediction of the critical heat flux of refrigerants flowing in heated, round microchannels has been developed and presented here. The model is based on the two-phase conservation equations and includes the effect of the height of the interfacial waves of the annular film. Validation has been carried out by comparing the model, a numerical solution of a non-linear system of five differential equations, with a critical heat flux (CHF) database including three different refrigerants from two different laboratories. More than 96% of the data are predicted within a ±20% error band and a mean absolute error of 8%. Furthermore, it is also possible to predict CHF data from a third laboratory for water and R-113 flowing in rectangular (using the width of the channel as the characteristic dimension) and circular microchannel heat sinks with multiple channels. All together, 90% of the entire database, including four different fluids and different geometries, are predicted by the model within a ±20% error band and a mean absolute error of 9.3% for channels from 0.215 to 3.15 mm in size, mass fluxes from 29 to 1600 kg/m² s, heated lengths from 10 to 126 mm and subcoolings from 2 to 77 °C.     

高温高压三维物理模型热损失控制及实验研究

高温高压注蒸汽三维物理模拟实验是进行稠油热采机理研究及优化油田开发方案的有效方法,模型是高温高压注蒸汽三维物理模拟实验装置的核心部分。为模拟真实油藏条件必须合理控制模型热损失。基于理论计算方法,采用热流数值模拟技术,提出了一种实验模型热损失控制设计方法。采用该方法,在氮气作为围压介质的大型高温高压三维物理模拟实验系统上实施了模型热损失控制方案。实验结果表明,模型热损失得到有效控制,油藏中蒸汽腔扩展得以真实模拟。     

Evaluation of a three-dimensional model for the prediction of heat transfer in power station boilers

This paper reports an evaluation of a full three-dimensional mathematical model of a power station boiler with emphasis on the heat transfer phenomena. The model is based on the numerical solution of the equations governing conservation of mass, momentum and energy and transport equations of scalar quantities. The radiative heat transfer is modelled using the discrete transfer method. The model was applied to a power station boiler of the Portuguese Electricity Utility where an experimental study was recently carried out. Measurements of gas temperature, species concentrations and incident heat fluxes to the walls were compared with the predictions for standard operating conditions, partial load operation and low excess air conditions. Comparison of the results with the measurements has shown that in general a good agreement was achieved, but in some cases only qualitative agreement was found. More detailed measurements would be needed to allow a better evaluation of the model and to identify the sources of discrepancy. Nevertheless, the model proved to be a useful tool for the analysis of the heat transfer in utility boilers.     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.     

Robust neuromorphic model for simultaneous prediction of convection heat transfer coefficient and friction factor of nanofluid flow in heat exchanging equipment

A robust neuromorphic model has been presented for simultaneous predictions of the Nusselt number and friction factor regarding nanofluid flow in heat exchanging devices. The most important influencing factors, namely, particle Peclet number, Reynolds number of the nanofluid, Prandlt number of the nanofluid, and volume fraction of the nanoparticles have been taken into consideration in the proposed models. The predicted results by the proposed model are much more accurate than the other available theoretical and semiempirical correlations. The presented computational technique is an accurate alternative for simultaneous prediction of Nusselt number and friction factor regarding the nanofluid-based heat exchanging equipments.     

Experimental methodology and heat transfer model for identification of ignition kinetics of powdered fuels

A methodology for investigating and quantifying the thermal processes leading to ignition of rapidly heated metal powders was developed. The simple experiment involves observing ignition of a powder coated on the surface of an electrically heated filament and is well suited for a variety of powdered fuels. In an experimental case study, the ignition temperature of spherical Mg powder was detected optically at different heating rates. To interpret the results, a heat transfer model was developed for a multilayer powder coating on the heated cylindrical filament. The thermal contact resistance between particles was determined from the measured bulk thermal diffusivity of the powder considering the experimental particle size distribution. An Arrhenius type expression was used to describe the exothermic chemical processes leading to ignition with the pre-exponent as an adjustable parameter. For Mg, a pre-exponent value identified by matching the calculations with the experimental data was found to be 10¹⁰ kg/m² s. The match between the experimental and predicted temperatures and times of ignition was good for different heating rates, which validated the proposed heat transfer model and indicated that the developed methodology is practically useful.     

An improved model for natural convection heat loss from modified cavity receiver of solar dish concentrator

A 2-D model has been proposed to investigate the approximate estimation of the natural convection heat loss from modified cavity receiver of without insulation (WOI) and with insulation (WI) at the bottom of the aperture plane in our previous article. In this paper, a 3-D numerical model is presented to investigate the accurate estimation of natural convection heat loss from modified cavity receiver (WOI) of fuzzy focal solar dish concentrator. A comparison of 2-D and 3-D natural convection heat loss from a modified cavity receiver is carried out. A parametric study is carried out to develop separate Nusselt number correlations for 2-D and 3-D geometries of modified cavity receiver for estimation of convective heat loss from the receiver. The results show that the 2-D and 3-D are comparable only at higher angle of inclinations (60° ? β ? 90°) of the receiver. The present 3-D numerical model is compared with other well known cavity receiver models. The 3-D model can be used for accurate estimation of heat losses from solar dish collector, when compared with other well known models.     

室温对全玻璃真空太阳集热管平均热损系数的影响

引言 平均热损系数（ULI）是判定全玻璃真空太阳集热管热性能的一个重要技术指标。它是指在无太阳辐照条件下，全玻璃真空太阳集热管内充满80℃热水时，通过真空集热管向周围环境传递热能，水温下降，管内平均水温与环境温度相差1℃时，吸热体单位表面积散失的功率。全玻璃真空太阳集热管的热损系数取决于选择性吸收表面的发射比大小和真空夹层内的气体压强，因此热损系数的大小可以综合地反映吸收表面发射比和夹层的真空度好坏，较主要地决定着真空集热管的高温性能。     

Numerical prediction of critical heat flux in pool boiling with the two-fluid model

Three-dimensional numerical simulations of the atmospheric saturated pool boiling are performed with the aim of predicting the critical heat flux. The two-phase mixture in pool boiling is described with the transient two-fluid model. The transient heat conduction in the horizontal heated wall is also solved. Dynamics of vapor generation on the heated wall is modeled through the density of nucleation sites and the bubble residence time on the wall. The heater’s surface is divided into zones, which number per unit area equals the density of nucleation sites, while the location of nucleation site within each zone is determined by a random function. The results show a replenishment of the heater’s surface with water and surface wetting for lower heat fluxes, while heater’s surface dry-out is predicted at critical heat flux values. Also, it is shown that the decrease of nucleation site density leads to the reduction of critical heat flux values. Obtained results of critical heat flux are in good agreement with available measured data. The presented approach is original regarding both the application of the two-fluid two-phase model for the prediction of boiling crisis in pool boiling and the defined boundary conditions at the heated wall surface.     

Simple model for thermal conductivity of nanofluids using resistance model approach

In this work thermal conductivity of the suspension is modeled using resistance model approach. This model considers Brownian motion and interfacial layer as well as a new mechanism where proposed by considering nanoparticles as liquid-like particles. The model can be used for estimation of upper and lower limits of nanofluid thermal conductivity, without any adjustable parameter. Thermal conductivity data of CuO nanofluids are obtained experimentally and the results show that the model is consistent to data. This model has been used for determination of various mechanisms' portions on thermal conductivity of nanofluids. The results are consistent with our knowledge about nanofluids' thermal conductivity mechanism.     

不同散热条件下微燃烧的数值模拟

散热是影响微尺度燃烧器燃烧稳定性的重要因素之一.本实验通过在一个长40 mm、内径2 mm、外径4 mm的石英玻璃直圆管表面施加不同的外部吹风温度,控制其表面散热.研究4、107、756℃外部风温下,微燃烧器的工作性能,其中燃料混合气体流量为0.16、0.28、0.32 L/min.实验测得燃烧器壁面温度,结合数值模拟研究内部燃烧过程.计算结果显示,提高燃料流量或外部风温可以提升反应强度、抑制熄火.如在风温107℃时,燃料气体当量配比下,当流量由0.16 L/min上升到0.32 L/min时,峰值温度由1538 K上升到1620 K;在流量0.28 L/min时,燃料气体当量配比下,当外部风温由4℃上升到756℃时,峰值温度由1592 K上升到1731K.     

The heat transfer and pressure loss characteristics of a heat exchanger for recovering latent heat (the heat transfer and pressure loss characteristics of the heat exchanger with wing fin)

In recent years the requirement for reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. A latent heat recovery type heat exchanger is one of the effective methods of improving thermal efficiency by recovering latent heat. This paper described the heat transfer and pressure loss characteristics of a latent heat recovery type heat exchanger having a wing fin (fin pitch: 4 mm, fin length: 65 mm). These were clarified by measuring the exchange heat quantity, the pressure loss of heat exchanger, and the heat transfer coefficient between outer fin surface and gas. The effects of condensate behavior in the fins on heat transfer and pressure loss characteristics were clarified. Furthermore, the equations for predicting the heat transfer coefficient and pressure loss which are necessary in the design of the heat exchanger were proposed. ©2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(4): 215–229, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20154     

A dynamic model for the efficiency optimization of an oscillatory low grade heat engine

A simple approach is presented for the modeling of complex oscillatory thermal-fluid systems capable of converting low grade heat into useful work. This approach is applied to the NIFTE, a novel low temperature difference heat utilization technology currently under development. Starting from a first-order linear dynamic model of the NIFTE that consists of a network of interconnected spatially lumped components, the effects of various device parameters (geometric and other) on the thermodynamic efficiencies of the device are investigated parametrically. Critical components are highlighted that require careful design for the optimization of the device, namely the feedback valve, the power cylinder, the adiabatic volume and the thermal resistance in the heat exchangers. An efficient NIFTE design would feature a lower feedback valve resistance, with a shorter connection length and larger connection diameter; a smaller diameter but taller power cylinder; a larger (time-mean) combined vapor volume at the top part of the device; as well as improved heat transfer behavior (i.e. reduced thermal resistance) in the hot and cold heat exchanger blocks. These modifications have the potential of increasing the relevant form of the second law efficiency of the device by 50% points, corresponding to a 3.8% point increase in thermal efficiency.     

COHEX: a computer model for solving the thermal energy exchange in an ultra high temperature heat exchanger. Part A: computational theory

The paper reports the development of a computer program that solves the thermal energy exchange and pressure drop characteristics for bayonet-element heat exchangers. The prime motivation for the study was to aid the design of a heat exchanger for the externally-fired combined cycle (EFCC) energy generation process. The essential feature of this high-efficiency process is the ceramic bayonet-tube gas–gas heat exchanger for use with shell-side temperatures up to 1600°C. It is envisaged that similar heat exchangers can be designed for applications in the metallic extraction and production industries. The program, named COHEX (Composite Heat Exchanger), solves the basic governing equations of the exchanger. It makes use of a numerical iterative approach from an initial tube-side outlet temperature estimate to converge to a solution. For given inlet conditions, the program evaluates the heat transfer between the shell-side and tube-side streams and arrives at the outlet conditions. This two-part paper presents a computational solution method using accepted techniques for conduction, convection and radiation in the ceramic heat exchanger. Part A addresses the technological background of the EFCC application and the theoretical content of COHEX in terms of accuracy and sophistication of the code. The second part of this paper, part B, describes the experimental facilities used to gather data in order to validate the program output. A comparison of computed and experimental data is presented. The paper progresses to illustrate the effects of parameter variation on heat exchanger output.     

快速冷却条件下凝固潜热处理模型的研究

深入分析了有限差分法模拟凝固过程时，采用等效比热法处理相变潜热的原理；从理论和实际计算角度分析了快速冷却条件下常规的等效比热法会造成热量“虚增”，模拟计算结果失真的原因和影响程度；并进一步建立了快速冷却条件下凝固潜热处理的模型。计算结果表明，该模型可以充分保证模拟计算的精度和效率。     

热系统内表面温度对空间有效温度场的影响

通过将平均辐射温度引入到建筑热舒适性传热计算中的方法简化建筑热舒适性和能耗的相互关系及其计算公式 ,进行平均辐射温度和房间的有效温度分布情况对热舒适性的影响分析。用这种方法来比较在不同热辐射换热和对流换热工况下变墙面温度时建筑内的热舒适性。理论分析和辐射、对流换热及其结合工况的热舒适性测试数据结果分析表明 ,热舒适性可以用平均辐射温度和有效温度来表征。用平均辐射温度和有效温度分析计算热建筑舒适性的方法 ,在优化建筑热系统的设计时 ,也有非常重要的参考价值。     

送风对中庭烟气温度的影响

通过理论计算,得出无喷淋中庭火源上的火焰气流各分区的烟气温度和速度随中庭高度变化曲线。在模型中庭内开展了一系列的送风发烟实验,并将自然状态下的实验结果与理论结果进行了比较,结果表明在紊流区两者相差不大。