空气冷却器结霜特性的实验研究

为研究空气冷却器结霜的影响因素,用实验的方法对冷风机的结霜现象进行研究。用显微照相法测量不同工况下的霜层厚度。结果表明:翅片间距、翅片表面材料、空气相对湿度、蒸发温度对冷风机的结霜过程有重要影响。翅片间距越小,结霜发生越早,霜层增长越快;翅片表面亲水材料推迟了结霜的发生时刻,但对结霜量无明显影响;蒸发温度越低,霜层增长越快,初期霜的粒径逐渐变细小;空气相对湿度越大,霜层的厚度越大,结霜速率越快,但结霜速率的增长趋势在相对湿度较高时有所减弱,相对湿度对结霜开始时间无明显影响。     

空气侧换热器结霜时传热与阻力特性研究

在质量、动量和能量守恒的基础上，采用分布参数法建立了空气侧换热器结霜时的动态数学模型，该模型耦合了结霜子模型和热交换子模型。利用该模型分析了不同温度和相对湿度下霜的厚度随时间的变化及结霜对空气侧换热器传热与阻力的影响。结果表明：在不同的工况下，空气侧换热器结霜情况不同；空气温度一定时，相对湿度越大，结霜越严重，融霜的时间间隔越短；随着结霜量的增加，换热器的换热量减小，风量也就逐渐减少，阻力却迅速增加。计算出了不同工况下融霜的时间间隔。     

翅片管式蒸发器结霜性能的仿真与实验研究

建立了结霜条件下翅片管蒸发器空气侧流动和换热的分布参数仿真模型,模型考虑了蒸发器结构、霜层厚度以及湿空气状态等参数在气流方向的沿程变化.对冰箱冷冻室蒸发器结霜条件下的动态性能进行了试验研究和数值模拟.结果表明,蒸发器结霜过程中的结霜量、能量传递系数和空气侧压降的计算值和试验值吻合良好,证明模型可以应用于翅片管蒸发器结霜性能的正确预测和优化设计分析.     

车载板翅式油冷却器的实验研究

某型车载板翅式油冷却器采用锯齿翅片强化油侧传热。利用风洞实验台对该油冷却器进行实验,测试其不同空气和油流量条件下的传热和流阻性能,并拟合了传热性能经验关联式。采用对数平均温差法建立油冷却器性能模型,以分析其空气侧部分遮挡工况下的实际性能。为提高油冷却器性能模型预测精度,借助FLUENT模拟得到了其锯齿翅片传热和流阻的特性数据。研究表明,正常工况下,油冷却器基于油侧表面积的总传热系数为49～74 W/(m2·℃),空气侧遮挡将显著降低油冷却器传热性能。     

风冷热泵冬季运行模拟与理论计算

建立了风冷热泵冬季运行状态下基于气液分相的集中参数模型。在模型中不仅对蒸发器干工况和结露工况作了数学描述，同时还分析了结霜过程中霜层密度和厚度变化对机组性能的影响，基于这一模型。选择某种热泵进行了计算，得到了这种上机组在不同温湿度和进出水温度下机组的能效比和结霜性能。     

石墨烯电热涂层在热泵机组抑霜/除霜上的应用研究

文中通过对室外换热器翅片冬季除霜的问题进行技术研究分析,在总结前人研究经验的基础上,改变换热器翅片的表面特性,利用环氧树脂石墨烯的疏水性能,并结合电加热的除霜技术制备出一种新型疏水性电加热涂层换热器。经测量证实了该涂层具有疏水性,在不同的电压及温度下对该涂层进行测试,发现不同含量的石墨烯对涂层的温升速率及温度稳定值呈正相关,并且该电加热涂层在低温工况下,电阻能够保持稳定,性能良好。另外,还在此基础上对该复合涂层的结霜融霜过程进行模拟,发现提高石墨烯含量可以抑制翅片的结霜速率,抑霜效果增强,通过与传统的电加热融霜方式模拟发现,复合涂层的融霜效果更加优越。     

肋片管式换热器过饱和假设下结霜数学模型的建立

在空气和霜层之间为过饱和空气假设的基础上，针对平肋肋片管式换热器的结霜过程，建立了数学模型，考虑了霜层密度变化和霜层阻力引起空气流量减小等因素，并将结霜模型与换热器传热特性模型有机地结合起来。     

气候环境实验室换热器除霜技术的研究

气候环境实验室在进行低温、高湿工况试验时,换热器表面会结霜。换热器结霜将增加换热空气的流动阻力、导致换热效率下降。为保证空气处理系统正常运行需要对换热器进行除霜。文中通过建立换热器模型对换热器结霜和除霜过程进行仿真分析,计算了结霜和除霜过程中霜层厚度、滞留水量的变化过程,提出了除霜开始判据和结束判据。以此为理论依据对实验室换热器除霜方案和除霜流程进行了优化,并在实验室调试过程中得到应用,取得了良好效果。     

霜增长对隔栅片管式热交换器性能的影响

霜增长对装有隔栅片的热交换器性能影响的实验研究业已完成。霜的累积、横贯热交换器的压力差和基于对数平均热函差(LMED)的能量转换系数,在作为空气湿度、空气面速度和叶片间距函数的结霜条件下被定量化。较高的空气湿度、空气面速度和较小的叶片间距都会导致结霜增长的加快、较高的压力差和较高的能量转换系数。因为热交换器上霜的堆积,总能量转换系数最终就会下降。这些趋势始终与这篇文献所报告的保持一致。     

低气压下翅片管换热器空气侧换热特性的实验研究

为研究低气压环境下翅片管换热器空气侧的换热特性,对不同气压环境下空气侧流速和翅片间距对平翅片管换热器空气侧换热特性的影响进行了实验分析。实验环境气压范围为40～100 kPa,换热器迎面风速为1.0～3.5 m/s,翅片间距2～3 mm。研究表明:实验工况下环境气压40 kPa时空气侧传热因子仅为常压下的30.42%～46.41%;低气压环境空气侧流速和翅片间距对空气侧换热的影响趋势与常压数据基本保持一致;不改变换热器结构,环境气压的变化仅影响空气物性,而对空气的流动状态的影响不大;翅片间距影响随Re的减小和环境气压的降低而减弱,两种翅片间距模型空气侧传热因子平均差异在环境气压为100 kPa时为12.07%,40 kPa时缩小为3.00%。     

Experimental investigation on the performance of air cooler under frosting conditions

Evaluation of heat transfer performance of the air cooler under frosting conditions is of great importance for the refrigeration industry. In this paper, effects of frost growth on the performance of the air cooler have been studied with an experimental air cooler of industrial size with different fin spacings, i.e., 6, 8 and 10 mm. Results showed that factors affecting the heat transfer performance of the air cooler include the evaporation temperature, the frost height, the fin spacing and the air velocity. The overall heat transfer coefficients based on a logarithmic mean temperature difference (LMTD) and the energy transfer coefficients based on a logarithmic mean enthalpy difference (LMED) were calculated. As the frost accumulated on the air cooler, the overall heat transfer coefficient and energy transfer coefficient will drop gradually.     

Frost behavior on a fin considering the heat conduction of heat exchanger fins

A mathematical model is proposed for predicting frost behavior on a heat exchanger fin under frosting conditions, taking into account fin heat conduction. The change in the three-dimensional airside airflow caused by frost growth is reflected in this model. The numerical estimates of frost thickness are consistent with experimental data, with an error of less than 10%. Due to fin heat conduction, frost thickness decreases exponentially toward the fin tip, while considerable frost growth occurs near the fin base. When a constant fin surface temperature is assumed, the predicted frost thickness was larger by more than 200% at maximum, and the heat flux by more than 10% on average, compared to results obtained with fin heat conduction taken into account. Therefore, fin heat conduction could be an essential factor in accurately predicting frost behavior. To improve prediction accuracy under the assumption of constant fin surface temperature, the equivalent temperature (for predicting frost behavior) is defined to be the temperature at which the heat transfer rate neglecting fin heat conduction is the same as the heat transfer rate with fin heat conduction taken into consideration. Finally, a correlation for predicting the equivalent temperature is suggested.     

Performance of finned tube heat exchangers operating under frosting conditions

In this paper, the performance of flat plate finned tube heat exchangers operating under frosting conditions was investigated experimentally. Heat exchangers of single and multiple tube row(s) were tested to show the effects of various parameters on heat transfer performance. The parameters include temperature and relative humidity of air, flow rate of air, refrigerant temperature, fin pitch, and row number. The time variations of heat transfer rate, overall heat transfer coefficient, and pressure drop of heat exchangers presented.     

Thermofluid characteristics of frosted finned-tube heat exchangers

The performance of frosted finned-tube heat exchangers of different fin types is investigated by experiments in this paper. The effects of the air flow rate, the air relative humidity, the refrigerant temperature, and the fin type on the thermofluid characteristics of the heat exchangers are discussed. The time variations of the heat transfer rate, the overall heat transfer coefficient, and the pressure drop of the heat exchangers are presented. The heat transfer rate, the overall heat transfer coefficient, and the pressure drop for heat exchangers with re-direction louver fins are higher than those with flat plate fins and one-sided louver fins are. The amount of frost formation is the highest for heat exchangers with re-direction louver fins.     

A method to reduce the flow depth of a plate heat exchanger without a loss of heat transfer performance

With the aim of improving heat exchanger compactness, this study investigates how the optimum configuration of an air–liquid plate heat exchanger changes as the heat exchanger depth decreases. In this respect, optimization of an air–liquid plate heat exchanger with a given frontal area and a given depth is achieved. The optimum fin pitch and plate pitch are obtained to maximize the heat transfer rate based on heat transfer and pressure loss correlations in finned channels. Then, the focus of this study is placed on how the optimum channel configuration changes when the heat exchanger depth decreases for compactness. The results illustrate that the heat transfer performance can remain unchanged if the geometric parameters, such as the plate thickness, the plate pitch, the fin thickness, and the fin pitch, are reduced proportionally to the square root of the flow depth reduction given that the flow remains laminar. This finding is arranged into a simple scaling rule to obtain the configuration of a more compact heat exchanger from an existing configuration. In addition, the scaling arguments are extended to practical situations where the fin thickness and the plate thickness are not properly reduced following the scaling rule due to limitations on available material thicknesses.     

Modeling for predicting frosting behavior of a fin–tube heat exchanger

A mathematical model is proposed to evaluate the frosting behavior of a fin–tube heat exchanger under frosting conditions. Empirical correlations of the heat transfer coefficients for the plate and tube surfaces and a diffusion equation for the frost layer are used to establish the model. The correlations for the heat transfer coefficients, derived from various experimental data, were obtained as functions of the Reynolds number and Prandtl number. The proposed model is validated by comparing the numerical results with experimental data for the frost thickness, frost accumulation, and heat transfer rate. The numerical results agree well with the experimental data. It is also found that this model can be applied to evaluate the thermal performance of a common fin–tube heat exchanger under frosting conditions.     

A three-dimensional numerical study and comparison between the air side model and the air/water side model of a plain fin-and-tube heat exchanger

CFD is becoming an important heat exchanger research technique. It constitutes an inexpensive prediction method, avoiding the need of testing numerous prototypes. Current work in this field is mostly based on air flow models assuming constant temperature of fin-and-tube surface. The purpose of this paper is to present an enhanced model, whose innovation lies in considering additionally the water flow in the tubes and the conduction heat transfer through the fin and tubes, to demonstrate that the neglect of these two phenomena causes a simulation result accuracy reduction.3-D Numerical simulations were accomplished to compare both an air side and an air/water side model. The influence of Reynolds number, fin pitch, tube diameter, fin length and fin thickness was studied. The exchanger performance was evaluated through two non-dimensional parameters: the air side Nusselt number and a friction factor. It was found that the influence of the five parameters over the mechanical and thermal efficiencies can be well reported using these non-dimensional coefficients. The results from the improved model showed more real temperature contours, with regard to those of the simplified model. Therefore, a higher accuracy of the heat transfer was achieved, yielding better predictions on the exchanger performance.     

汽轮发电机气体冷却器性能试验研究

气体冷却器是汽轮发电机的重要设备之一。其传热与阻力性能将直接影响汽轮发电机的运行经济性和可靠性。为实现汽轮发电机气体冷却器的优化设计,对不同翅片间距的翅片管冷却器的传热和阻力性能进行了试验研究,得到了Re在3 000~190 000之间换热器翅片侧的传热和阻力特性,并分析了风速和翅片间距对气体冷却器传热和阻力性能的影响规律。研究成果对汽轮发电机气体冷却器的结构与性能优化具有重要的指导作用。     

Peripheral fins for blockage robustness

A peripheral finned-tube, cross-flow heat exchanger (evaporator) is briefly introduced that allows for uninterrupted and effective air flow in the presence of condensate or frost. The peripheral fins are connected to tubes with radial fins and the surface areas of both radial and peripheral fins allow for surface-convection heat transfer. The peripheral fins have a staggered arrangement to allow for alternate air flow paths in the presence of a blockage. Optimized fin structure is sought using one-dimensional fin models. The peripheral fins allow for significant surface-convection by using the stagnation–flow regions as well as the boundary–layer break ups. The CFD results show that the peripheral fins mitigate the pressure drop penalty due to blockages and in this regard present an advantage over the conventional fins. CFD results show that fin pitch can be optimized. The anisotropy of the peripheral fin structure may also allow for easy drainage of the condensate along the tubes when tubes are along gravity.     

百叶窗翅片结构对空气侧流动换热影响的研究进展

百叶窗翅片作为换热器主要翅片形式之一,其结构对空气侧流动换热特性有着重要影响。本文总结了近年来国内外在百叶窗翅片结构对空气侧流动换热影响方面的研究,包括翅片间距、翅片高度、翅片厚度、翅片深度、百叶窗间距及开窗角度对空气侧换热系数、压降、流动效率、传热因子和摩擦因子的影响。研究表明:传热因子随开窗角度和翅片深度的增加而增大,随翅片间距的增加而减小;摩擦因子随开窗角度的增大而增大,随百叶窗间距的增大而减小;其中,开窗角度与翅片深度分别是影响空气侧流动和换热的最主要因素。最后,在百叶窗结构的基础上,提出了对翅片表面进行处理以及使用新型翅片结构等途径来进一步强化空气侧流动换热的建议。