The effects of design and operating factors on the frost growth and thermal performance of a flat plate fin-tube heat exchanger under the frosting condition

An experimental study of the effects of various factors (fin pitch, fin arrangement, air temperature, air humidity, and air velocity) on the frost growth and thermal performance of a fin-tube heat exchanger has been conducted under the frosting condition. It is found that the thermal performance of a heat exchanger is closely related to the blockage ratio of the air flow passages due to the frost growth. The maximum allowable blockage ratio is used to determine the criteria for the optimal operating conditions of a fin-tube heat exchanger. It is also shown that heat transfer rate of heat exchanger with staggered fin arrangement increases about 17% and the time required for heat transfer rate to reach a maximum value becomes longer, compared with those of an inline fin-tube heat exchanger under the frosting condition. The energy transfer resistance between the air and coolant decreases with the increase of inlet air temperature and velocity and with decreasing inlet air humidity.     

A study on energy saving of cooling/reheating system using compact heat exchanger

When circulated air passes through the cooling coil in an air-conditioning system, the air is over-cooled to eliminate the moisture and decrease the temperature. The cooled air is then reheated to recover the temperature. The purpose of the present study was to evaluate the performance of a cooling/reheating system with regard to both cooling and reheating energy savings affected by exchanging heat between the cooled air and the reheated air with a compact heat exchanger. The thermal and dehumidification behaviors of the system were evaluated experimentally and then compared with simulation data. The results show that the energy saving rate was as high as 50% under the present experimental conditions and was affected by the face velocity of the heat exchanger, the inlet temperature, the inlet humidity ratio, and the effectiveness of heat exchanger. Furthermore, the experimental data were found to be in fairly good agreement with the simulation data.     

基于高分子透湿膜的全热回收器的试验研究

利用透湿膜设计制造了一台平行流道结构的全热换热器,并在焓差实验室对其进行性能测试。试验研究了室内侧的干球温度27℃,湿球温度19℃的条件下,迎面风速变化、新风温度变化、新风相对湿度变化时对全热换热器的显热效率、潜热效率、阻力的影响。实验研究表明,在较高的迎面风速的条件下,全热换热器仍然具有较高的显热效率和潜热效率。显热效率和潜热效率分别在70%～85%和60%～75%之间。温度和湿度变化对显热效率的影响很小,而对潜热效率的影响很大。随着新风的温度和相对湿度的增大,潜热效率也随着增大。在炎热潮湿的气候条件下,基于高分子透湿膜的全热回收器具有较高的显热效率和潜热效率,表明了这种换热器在南方地区具有较好的应用前景。     

微型风扇出口速度分布及其对散热的影响

对两种不同型号微型风扇出口速度进行了测量,并对这两种型号风扇出口速度分布进行了比较。在此基础上,对散热器散热量进行了计算,得到了散热器出口的空气温度分布,并比较了采用两种微型风扇时散热器的散热量,以及相同空气流量、不同空气入口速度分布条件下散热器的散热量。计算结果表明,在相同空气流量条件下,均匀的速度分布有助于提高散热器的散热量。     

Combined heat and mass transfer analysis during the dehumidification process within a plate type heat exchanger

Liquid desiccant systems have been paid attention because of its advantages in energy saving and an environmental friendliness. The use of liquid desiccant systems offers design and performance advantages over the solid desiccant systems, especially when solar energy is used for regeneration. The objective of this paper is to analyze the simultaneous heat and mass transfer characteristics of lithium chloride aqueous solution for the plate type dehumidification system. The effects of process air and solution inlet conditions on the dehumidification performance are studied in this study. It is found that the heat transfer coefficient of the air side gives much more significant effect on the absorption rate and dehumidification effectiveness than those of the solution and the coolant sides while the mass transfer coefficient of the solution side gives more significant effect than that of the air side. It is also found that the solution concentration is the most important factor for absorption performance improvement during the dehumidification process.     

用于独立新风换气机的膜式全热换热器研究进展

利用独立新风换气机回收空气余热是一种重要的建筑节能技术。膜式全热换热器具有节能、高效、结构简单等优点,已被用作新风换气机的核心部件。本文首先介绍了膜式全热换热器的特点,然后综述了芯体材料、芯体结构、传递模型、强化传热和应用等方面的国内外研究进展,最后对其研究和发展前景提出了展望。     

Moisture transfer characteristics of a LiCl-impregnated paper membrane used for an enthalpy exchanger

In this study, moisture diffusion coefficient of a LiCl-impregnated paper membrane was experimentally obtained through an analysis of heat and moisture transfer of an enthalpy exchanger made with the membrane. A sorption curve was separately obtained by a weighing method. Tests were conducted as a function of relative humidity and temperature. Diffusion coefficient first increases up to 60% relative humidity and then decreases. It also increases with the increase of temperature. The variation of the diffusion coefficient is in the range of 1.8 x 10^?11 m²/s to 6.0 x 10^?11 m²/s, which is smaller than reported values of hydrophilic polymers. The variation of the moisture permeability, however, is in the range of 2.4 x 10^?7 kg/ms to 1.2 x 10^?6 kg/ms, which is comparable with reported values of hydrophilic polymers. Thus, similar moisture transfer performance is expected between LiCl-impregnated paper and polymer enthalpy exchangers. Possible explanation on the behavior of the diffusion coefficient with relative humidity is provided.     

环状与蜂窝蓄热体传热与阻力特性试验研究

蓄热换热器具有结构简单、造价低、效率高等优点,在余热回收方面有着广阔的应用前景。蓄热体作为蓄热换热器的关键部分,其形状、大小及材质等各项性能参数都会对蓄热系统的余热回收效果产生重要的影响。对环状和蜂窝这两种蓄热体的传热与阻力特性进行了试验研究,通过对试验结果分析,得出了热空气进口温度和速度对蓄热体冷热空气出口温度、传热速率、温度效率、热效率及阻力损失的影响规律,并从温度效率、热效率和阻力损失几方面比较了两蓄热体的传热与阻力性能,发现在低进口速度时环状蓄热体的综合性能更好。     

煤矿井下空调蒸发器对流传热特性试验

对恒定控制制冷剂R407C工作温度为9℃、额定制冷量为45k W条件下的蒸发器进行了模拟矿井条件湿空气的对流传热试验。研究结果表明:蒸发器的制冷量随进口空气温度的升高而增加,随进口空气相对湿度的增加而增加;出口空气温降随进口空气温度的升高而增加,随进口空气相对湿度的增加而缓慢降低;对流传热的Nu数随Re数的增加而增加,随进口空气相对湿度的增加而降低。     

干湿工况下波纹翅片管换热器空气侧特性的对比

对7个带亲水层和3个不带亲水层波纹翅片管换热器在析湿工况下空气侧的换热压降特性进行了试验研究,在不同的入口风速和入口相对湿度下比较了干湿工况下的空气侧特性。结果表明,对带亲水层的波纹翅片,析湿工况下的压降显著地高于干工况下的;当入口风速小于0.5 m/s时,湿工况下的换热性能低于干工况下的,当入口风速增加到2.0 m/s时,湿工况下的换热性能则强于干工况下的。析湿工况下带亲水层波纹翅片的换热性能比不带亲水层波纹翅片的偏低一些,但是表面涂上亲水层可以大大降低空气侧的压降。提出析湿工况下波纹翅片管换热器空气侧的换热和压降关联式,平均误差分别为8.70%和7.90%,可用于设计波纹翅片管换热器或者评价它的性能。     

空气源热泵蒸发器结霜及换热性能的研究

研究了入口空气相对湿度、温度以及风速对风冷热泵蒸发器结霜的影响,获得了各个入口空气参数对蒸发器结霜厚度和换热量的影响规律.研究发现空气入口相对湿度等参数对蒸发器结霜以及换热性能有很大的影响,结霜严重地影响了蒸发器换热性能.计算结果和实验测试的结果吻合良好.     

Heat transfer and pressure drop amidst frost layer presence for the full geometry of fin-tube heat exchanger

The present study numerically solves the flow and thermal fields in the full geometry of heat exchanger modeling with frost layer presence on the heat exchanger surface. The effects of air inlet velocity, air inlet temperature, frost layer thickness, fin pitch, fin thickness, and heat exchanger shape on the thermo-hydraulic performance of a fin-tube heat exchanger are investigated. Heat transfer rate rises with increasing air inlet velocity and temperature, and decreasing frost layer thickness and fin pitch. Pressure drop rises with increasing air inlet velocity and frost layer thickness, and decreasing fin pitch. The effect of fin thickness on heat transfer and pressure drop is negligible. Based on the present results, we derived the correlations, which express pressure drop and temperature difference between air inlet and outlet as a function of air inlet velocity and temperature, as well as frost layer thickness.     

有机朗肯循环系统动态响应分析

现有的有机朗肯循环动态仿真,多是以分布参数法或移动边界法构建换热器的动态模型,以致仿真速度较慢。基于此,提出一种新方法,构建换热器的动态模型,此模型利用四个参数表征换热器特性,减少了动态方程的数目,提高了求解速度,广泛适合于工程计算中的不同类型换热器。利用此模型比较了不同换热能力和不同储液体积的换热器的动态响应情况以模拟比较不同类型换热器的动态性能,模拟结果显示换热能力越好,内部储液体积越少的换热器,出口响应越快。并且在Matlab/Simulink平台上搭建了ORC系统的动态模型,模拟了在入口热源温度波动下的ORC系统运行状况,并与试验数据进行了验证,在保证较快的计算能力的同时,达到满意的计算精度,模拟结果与试验误差达到5.8%。最后讨论了调节流量对于抵消出口热源温度波动的作用,发现尽快调节流量可稳定出口焓值,抵消热源温度变化所导致的波动,但在一定时间之后再调节则对稳定出口焓值的意义不大。为提出控制手段提供参考。     

干盘管系统冷冻水流量调节试验研究

通过对独立新风加干式风机盘管空调系统在不同室外工况下的试验研究,分析了冷冻水流量调节对空调系统性能及室内温湿度影响。结果表明,随冷冻水流量增加,系统提供的显热制冷量及潜热制冷量增加,而提供的冷量显热比减少;当冷冻水不能满足新风机组制冷要求时,增加水流量可以明显降低室内冷量显热比,在冷冻水流量满足新风机组制冷要求的基础上增加水流量,对新风机组的制冷量及室内显热比的影响不大;增加冷冻水流量对室内温度影响较大,而对室内相对湿度影响较小。     

毛细管平面辐射空调系统的优化控制

针对传统空调系统中主要通过对流换热的方式来消除室内的热湿负荷造成能源品味上的浪费问题,将毛细管辐射换热技术应用于空调系统中,开展了由毛细管承担室内显热负荷而由新风系统承担全部的室内潜热负荷空调处理方案的分析,建立了空调房间温度与湿度之间相互独立的控制方法,并提出了采用冷冻水变流量优化控制法解决毛细管换热中的结露问题.在新风系统运行时,毛细管辐射供冷系统停止供水;新风系统除湿结束后,毛细管辐射供冷系统开始供水,在此基础上对毛细管辐射供冷系统采用间歇性流量控制方案进行了评价.研究结果表明,该控制方案不仅简单易行,而且节约能源.     

Thermal performance analysis of heat exchanger for closed wet cooling tower using heat and mass transfer analogy

In closed wet cooling towers, the heat transfer between the air and external tube surfaces can be composed of the sensible heat transfer and the latent heat transfer. The heat transfer coefficient can be obtained from the equation for external heat transfer of tube banks. According to experimental data, the mass transfer coefficient was affected by the air velocity and spray water flow rate. This study provides the correlation equation for mass transfer coefficient based on the analogy of the heat and mass transfer and the experimental data. The results from this correlation equation showed fairly good agreement with experimental data. The cooling capacity and thermal efficiency of the closed wet cooling tower were calculated from the correlation equation to analyze the performance of heat exchanger for the tower.     

Numerical simulation of the effects of a suction line heat exchanger on vapor compression refrigeration cycle performance

Most modern refrigerators incorporate heat transfer between the refrigerant in a capillary tube and the refrigerant in a suction line. This heat transfer is achieved by a non-adiabatic capillary tube called a capillary tube-suction line heat exchanger and is supposed to improve the performance of the small vapor compression refrigeration cycle by removing some enthalpy of the refrigerant at the evaporator entrance. To investigate the effects of this heat transfer on the refrigeration cycle, a computer program was developed based on conservation equations of mass, momentum, and energy. The non-adiabatic capillary tube model is based on a homogeneous two-phase flow model. The simulation results show that both the location and length of the heat exchange section influence the coefficient of performance (COP) as well as the cooling capacity. It is noteworthy that the influence was not monotonic; that is, the performance may be deteriorated under certain conditions.     

开孔折流板对列管式换热器传热性能的影响研究

针对由传统弓形折流板结构带来的壳程流动死区,从而引起的流动阻力大、传热效率低等问题,本文对折流板进行开孔,通过数值模拟的方法研究开孔折流板结构对列管式翅片换热器壳侧流体流动、传热及阻力性能的影响。研究发现,折流板开孔后,壳程流动死区明显减小,壳程传热系数及压降同比开孔前降低了;综合换热性能同比开孔前提升了。壳程压降随开孔率及板间距的增大而减小,壳程努塞尔数Nu随板间距的增大逐渐增大。从综合换热性能及场协同的角度分析发现,开孔率x=0.229、折流板间距H=85 mm的列管式换热器综合传热性能最佳。     

管壳式换热器设计及软件开发

传统管壳式换热器的设计工作量大、效率低,设计方法比较粗糙,设计偏差较大。本文基于VB6.0编程语言,使用分段设计方法,开发出了管壳式换热器设计软件,该设计方法将换热器按照管壳程结构分为有限个单元,根据工艺条件,求解单元温度分布,依次对每个单元进行设计,进而完成整台换热器的设计,该方法充分考虑了介质物性随温度变化对设计的影响,设计结果与HTRI对比,误差很小。本软件可根据需要进行选型设计,也可以以总传热系数与总压降的比值K/ΔP为评价标准,选取性能最高的换热器结构,完成优化设计。     

Heat transfer characteristics in latent heat storage system using paraffin wax

An energy storage system has been designed to study the heat transfer characteristics of paraffin wax during melting and solidification processes in a vertical annulus energy storage system. In the experimental study, three important issues are focused. The first one is temperature distribution in the phase change material (PCM) during the phase change processes. The second one is the thermal characteristics of the paraffin wax, which includes total melting and total solidification times, the nature of heat transfer phenomena in melted and solidified PCM and the effect of Reynolds number as inlet heat transfer fluid (HTF) conditions on the heat transfer parameters. The final one is to calculate heat transfer coefficient and effectiveness during solidification process. The experimental results proved that the PCM melts and solidifies congruently, and the melting front moved from the top to the bottom of the PCM container whereas the solidification front moved from bottom to the top along the axial distances in the PCM container. Experiment has been performed for different water flow rates at constant inlet temperature of heat transfer fluid for recovery and use of heat. Time-based variations of the temperature distributions were explained from the results of observations of melting and solidification curves. Charging and discharging processes were carried out. Heat transfer characteristics were studied.