Effects of subcooling degree and injection super‐heating degree on performance of an air source heat pump with subcooler

For a heat pump, three injection styles including liquid, two‐phase, and vapor injection are used to improve its performance. But there is no quantitative index to define the injection styles. They are distinguished from the injection configurations. The subcooler refrigerant injection (SCRI) is generally considered as the vapor injection. In this study, the injection super‐heating degree (ΔT_inj) is introduced as a technical parameter to define the injection style. The ΔT_inj is affected by the subcooling degree (ΔT_sc). These two parameters are used to study their effects on the heat pump performance. Experimental and theoretical analysis reveals that three injection styles can be selectively achieved in the SCRI configuration by the suitable control of the main and subcooling electronic expansion valves. Additionally, for liquid and two‐phase injection, it can improve heating capacity (Q_h) by heightening the ΔT_sc to increase the heat exchange amount in the evaporator. For vapor injection, in addition to increase of the ΔT_sc, the Q_h can be enhanced by raising the ΔT_inj to lift the compression power of the compressor. The two‐phase injection is more beneficial to enhance the heat pump performance than the liquid injection and two‐phase injection. The largest Q_h and corresponding increase amplitude (δQ_h) were, respectively, obtained with two‐phase injection to be 13 577 W and 13.5%, and the corresponding COP and increase amplitude (δCOP) were, respectively, 2.36 and 16.8%.     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

The impact of fouling on performance evaluation of multi-zone feedwater heaters

A numerical method for analyzing closed system feedwater heaters is presented. A general approach to determine area allocations among the desuperheating, condensing and subcooling zones under a known set of operating conditions is presented for a feedwater heater in a steam power plant. A significant amount of heat duty is handled by the condensing zone, whereas the subcooling zone handles a least amount of heat duty which essentially vanishes at low steam pressures. Fluids mass flow rates and accordingly the overall heat transfer coefficients have significant effects on the areas needed for desuperheating, condensing and subcooling in a feedwater heater. Two fouling models are considered to examine their effect on the heat exchanger performance. Insignificant changes were noticed when comparing the heat transfer rate and outlet temperature results of both the models. It is found that heat duty of the heat exchanger decreases by 2.7% in 3 years when we use the recommended fouling resistance, while the outlet shell-side fluid temperature increased by 6.3%.     

Exergy analysis of latent heat storage systems with sensible heating and subcooling of PCM

The effect of sensible heating of the phase-change material (PCM) before melting and subcooling after solidification, on the performance of the latent heat storage system (LHSS), is studied in terms of first- and second-law efficiencies for the overall charge–discharge cycle. The external heat transfer irreversibilities on account of the interaction between the heat transfer fluid and the storage element are characterized and the optimum phase-change temperature for maximum second-law efficiency is studied. The performance of the LHSS operation is assessed with and without sensible heating before melting and subcooling after solidification. It is observed that the optimum phase-change temperature is higher, and, the overall second-law efficiency is greater for LHSS with sensible heating and subcooling beyond a certain phase-change temperature compared to latent heat storage alone. In addition, the first-law overall efficiency is found to exhibit a minimum and a pre-heated discharge stream is shown to result in a substantial improvement of the second-law efficiency. © 1998 John Wiley & Sons, Ltd.     

冷介质通过井筒注入某地质结构的流动与传热预测软件研发

通过井筒向某地质结构内注入冷介质时,由于地温与冷介质之间存在温差,冷介质将通过井筒结构与土壤进行热量交换,最终导致冷介质冷量损失进而温度升高而达不到所需温度要求。因此,在施工前,需要对井筒进行冷量损失和沿井筒的温度分布的预测,为保冷结构设计以及井筒结构材料选型提供数据支持。由于注入冷介质的过程中,热量交换过程是一个非稳态传热过程,通常只能采用数值模拟来进行预测。为了简化数值模拟复杂的计算过程,采用准稳态的传热方法来构建单相冷介质通过井筒注入时的流动换热的物理数学模型,并开发了一个数值仿真软件。将仿真结果同商业软件FLUENT模拟结果进行了比较,表明温度分布和冷量损失基本一致,由此验证了所提模型的正确性和可靠性,为非工程热物理专业的工程技术人员提供能够预测低温工质在井筒流动与换热过程的仿真软件。     

Pool boiling of perfluorocarbon mixtures on silicon surfaces

The need for higher pool boiling critical heat flux (CHF) in electronic cooling applications has turned attention to the use of binary mixtures of dielectric liquids. The available literature demonstrates that the addition of a liquid with higher saturation temperature, higher molecular weight, higher viscosity and higher surface tension can lead to significant enhancement of CHF, beyond what can be achieved through changes in pressure, liquid subcooling, and the product of surface effusivity and heater thickness. The current study focuses on extending the available data on mixture CHF enhancement, as well as pool boiling, on polished silicon surfaces to FC-72/FC-40 mixture ratios of 10%, 15%, and 20% of FC-40 by weight, a pressure range of between 1 and 3 atm, and fluid temperature from 22 to 45 °C, leading to high subcooling conditions. It is found that peak heat flux can be increased to as high as 56.8 W/cm² compared to 25.2 W/cm² for pure FC-72 at 3 atm and 22 °C. It is believed that the increase in the mixture latent heat of evaporation and surface tension, accompanying the depletion of the lower boiling point fluid in the wall region plays the major role in enhancing the critical heat flux for binary mixtures.     

Research on Marangoni condensation heat transfer for water and ethanol mixture vapor

A brass block was constructed as a test block to study the Marangoni condensation in this paper. The maximal temperature difference of the block surface on which pure steam condensed was 11^°C when the block was cooled by the normal temperature water. Regulations and modes of Marangoni condensation for mixture vapor with different mass fractions were studied when the speed of vapor was 0.3 m/s. As both temperature gradients and concentration gradients exist on the condensing surface, the experimental results indicate that the maximal heat transfer coefficient of mixture vapor can be 2.8 times that of pure steam when the Marangoni condensation of mixture vapor appears. The heat transfer coefficient of mixture vapor increases with the decrease of surface subcooling, and it appears a steep increase when the surface subcooling is small enough; the heat transfer flux has a maximum value as the surface subcooling rises; and the different modes of condensation are confirmed when the different ethanol concentration and different surface subcooling exist. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 505–514, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20035     

Numerical study of heat transfer of hydrogen combustion in noble gases atmosphere in compression ignition engine

The high energy content of hydrogen and zero carbon emission from hydrogen combustion is very important for compression ignition engine development. Hydrogen requires a very high auto-ignition temperature, which encourages replacing nitrogen with noble gases with higher specific heat ratio during compression process. In noble gases-hydrogen combustion, higher combustion temperature potentially leading to a higher heat loss. This paper aims to investigate the effect of hydrogen combustion in various noble gases on heat distribution and heat transfer on the cylinder wall. Converge CFD software was used to simulate a Yanmar NF19SK direct injection compression ignition engine. The local heat flux was measured at different locations of cylinder wall and piston head. The heat transfer of hydrogen combustion in various noble gases at different intake temperatures was studied using the numerical approach. As a result, hydrogen combustion in light noble gases such as helium produces faster combustion progress and higher heat temperature. The hydrogen combustion that experienced detonation, which happened in neon at 340 K and argon at 380 K, recorded a very high local heat flux at the cylinder head and piston due to the rapid combustion, which should be avoided in the engine operation. At a higher intake temperature, the rate of heat transfer on the cylinder wall is increased. In conclusion, helium was found as the best working gas for controlling combustion and heat transfer. Overall, the heat transfer data gained in this paper can be used to construct the future engine hydrogen in noble gases.     

Theoretical Analysis of Optimal Subcooling for Single Vapor-Compression Refrigeration Systems

A finite temperature difference heat transfer method and irreversibility analysis have been developed for investigating the effects of subcooling on coefficient of performance, cooling water pressure drop of condenser, and heat exchanger area for R1234yf, R1234ze, R22, R134a, and R410A in a single vapor-compression refrigeration system. In order to satisfy the increasing cooling load for subcooling in a condenser, the heat exchanger size or cooling water pumping power that corresponds to initial cost or operating cost, respectively, is increased. The optimum degree of subcooling in a refrigeration system with superior performance and least initial cost or operating cost is obtained numerically. The results show that the maximum coefficient of initial cost saving and coefficient of operating cost saving and their corresponding optimum degree of subcooling increase with condensation temperature. At a higher inlet temperature of cooling water, the optimum degree of subcooling turns out to be smaller for all refrigerants. The results are expected to facilitate the prospective design of a vapor-compression refrigeration system for using alternative refrigerants.     

Deterioration of heat transfer to supercritical helium at 2·5 atmospheres

Heat transfer has been investigated for supercritical helium at 2·5 atm flowing inside a vertical tube with inlet bulk fluid temperatures less than the transposed critical temperature. Results indicate that for high heat flux conditions, the heat-transfer coefficient passes through a maximum and then deteriorates as the fluid temperature approaches the transposed critical temperature. This is contrary to the predictions of a correlation developed in an earlier study of supercritical helium heat transfer under low heat flux conditions, which only predicts enhancement in heat transfer as the transposed critical temperature is approached.The experimental data are presented and conditions under which heat-transfer deterioration was observed are discussed. The probable limitations to the validity of the above mentioned heat-transfer coefficient correlation, developed for a different range of experimental data, are also discussed.     

Film cooling with helium injection into an incompressible air flow

An experimental study of film cooling effects produced by injection of a helium secondary flow into a low speed air mainstream is presented. The helium was injected through a porous section into a turbulent boundary layer flowing over a flat plate. For comparison, measurements were also made using air as the secondary fluid.

The adiabatic wall temperature, presented in a dimensionless group known as the film cooling effectiveness, was measured for both helium and air secondary flows. The experimental data are compared with predictions. It is found that helium injection produces higher film cooling effectiveness than anticipated. The measured helium concentration along the wall is in relatively good agreement with predictions.

Temperature and helium concentration profiles are also presented. The concentration and temperature profiles are very similar and in good agreement with profiles measured with air injection.     

Nucleate Boiling Enhancements on Porous Graphite and Microporous and Macro–Finned Copper Surfaces

Enhancements in nucleate boiling heat removal with dielectric liquids, by increasing either the bubbles nucleation sites density and/or the wetted surface area, are desirable for immersion cooling of high-power computer chips. This article presents the results of recent investigations of nucleate boiling enhancement of FC-72, HFE-7100, and PF-5060 dielectric liquids on porous graphite, copper microporous surfaces, and copper surfaces with square corner pins, 3 mm × 3 mm in cross-section and 2, 3, and 5 mm tall. All surfaces have a footprint measuring 10 × 10 mm. These investigations examined the effects of liquid subcooling up to 30 K and surface inclination, from upward-facing to downward-facing, on nucleate boiling heat transfer coefficient and critical heat flux. Natural convection of dielectric liquids for cooling chips while in the stand-by mode, at a surface average heat flux <20 kW/m², is also investigated for the different surfaces.     

地下热流固耦合对EGS热开采的影响

人工热储的孔隙率及渗透率在增强型地热系统（EGS）地下热开采过程中受温度（T）、水力（H）、应力（M）的综合影响。本文建立了EGS热开采过程THM耦合的三维计算模型,并采用局部非热平衡假设处理液岩对流换热。对一理想的五口井EGS系统采热过程进行了THM模拟计算,分析了岩石温度、孔隙压力对岩石应力场的作用机理,进一步研究了应力场对EGS采热性能的影响。结果表明,开采过程中岩石应力场为热储内孔隙压力和温差综合作用的结果,由孔隙压力造成的岩石应力为压应力,仅集中于注入井附近,由岩石温度变化引起的热应力为拉应力,随着热开采区域的扩展而扩展。液−岩温差是触发工质与岩石热交换的动因,同时也是产生热应力的根本。     

Temperature measurements near a heating surface at high heat fluxes in subcooled pool boiling

In previous papers (Int J Heat Mass Transfer, 2008;50:3481–3489, 2009;52: 814–821), the authors conducted measurements of liquid–vapor structures in the vicinity of a heating surface for subcooled pool boiling on an upward‐facing copper surface by using a conducting probe method. We reported that the macrolayer dryout model is the most appropriate model of the CHF and that the reason why the CHF increases with increasing subcooling is most likely that a thick macrolayer is able to form beneath large vapor masses and the lowest heat flux of the vapor mass region shifts towards the higher heat flux. To develop a mechanistic model of the CHF for subcooled boiling, therefore, it is necessary to elucidate the effects of local subcooling on boiling behaviors in the vicinity of a heating surface. This paper measured local temperatures close to a heating surface using a micro‐thermocouple at high heat fluxes for water boiling on an upward‐facing surface in the 0 to 40 K range of subcooling. A value for the effective subcooling, defined as the local subcooling during the period while vapor masses are being formed was estimated from the detected bottom peaks of the temperature fluctuations. It was established that the effective subcooling adjacent to the surface remains at considerably lower values than the bulk liquid subcooling. This suggests that, from nucleation to coalescence, the subcooling of a bulk liquid has a smaller effect on the behavior of primary bubbles than the extent of the subcooling would appear to suggest. An empirical correlation of the effective subcooling is proposed to provide a step towards quantitative modeling of the CHF for subcooled boiling. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20277     

Upper bound and best estimate of the efficiency of the iodine sulphur cycle

An upper bound and a best estimate of the efficiency of a thermochemical iodine sulphur water splitting cycle coupled with a High Temperature Reactor operating between 760 K and 1144 K are proposed in this publication. An upper bound of 51% is found assuming ideal reversible chemical reactions and constraints related to the heat exchange between helium and chemical process. It appears that the separations between water and sulphuric acid on one hand and water, HI and iodine on the other hand are the most energy consuming. The best estimate of the efficiency is found between 33% and 36% according to the assumptions made for the temperature pinch at the heat exchangers of the hydrogen production section. More realistic values can be obtained after having designed the heat exchangers of this section.     

Formation enhancement of methane hydrate for natural gas transport and storage

Methane hydrate is considered an excellent way of transporting and storing natural gas in large quantities. However, when methane hydrate is formed artificially, water/gas ratio is relatively low due to a slow reaction rate between water and methane gas. The major objective of this study is to investigate the mechanics of methane hydrate formation and to explore possible means for rapid production of hydrates and increasing its water/gas ratio. It is found that methane hydrate could be formed rapidly during pressurization if the subcooling is maintained at 8 K or above. In addition, water injection appears to be more effective in hydrate formation compared to gas injection or using a magnetic stirrer. It also gives higher water/gas ratios of 3–4 times for the methane hydrate through a nozzle at the same level of subcooling temperature, when compared to gas injection cases.     

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

ABSTRACT

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling, and mass fluxes are considered. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 microns. The channel is heated on one side, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface and the other was a surface coated with zinc oxide nanostructures that are superhydrophilic. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter, as indicated by reduced temperature oscillations during boiling, and achieving higher maximum heat flux without dryout. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm² with an average surface temperature of 134° Celsius. Recommendations for electronics cooling applications are also discussed.     

液氮、液氢加注系统数值仿真与试验研究

以低温液氮加注系统为研究对象,通过FLUENT仿真与试验研究,结果表明：数值仿真与试验结果吻合良好,小流量预冷液氮温度升高约5 K,一定过冷度与低漏热率对单相加注具有重要作用。以低温储液瓶为研究对象,对“加注过程”进行仿真与试验,结果表明：加注初期液位高度低于临界液位,加注对整个流场扰动较大,加剧了储液瓶内部对流换热,液相区有气泡形成,部分气相二次液化,瓶内暂无温度分层;加注中后期,气液交界面趋于稳定,液相区无明显气泡,气相区无明显二次液化且温度逐渐形成分层;数值仿真与试验的液氮蒸发率分别约为1.7%和1.5%,具有较好一致性。基于液氮分析经验,对液氢“加注过程”进行仿真预测,结果表明：液氮、液氢加注过程储液瓶内部液相率与温度具有相似变化规律,液氮加注分析对液氢加注具有一定参考意义。     

Boiling heat transfer characteristics in liquid–liquid direct contact parallel flow of immiscible liquids

Boiling heat transfer characteristics in liquid–liquid direct contact parallel flow of immiscible liquids were investigated. The effect of operating conditions and column specifications on the UA factor of heat transfer were determined and the following empirical correlation is proposed for conditions where subcooling is constant: . The Stanton number for heat transfer to the dispersed phase correlates with the product of the Jacob number and the Prandtl number, H^*. A comparison of heat transfer-flow pattern and the correlation equation for heat transfer show good agreement within a range of ±30%. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(8): 493–514, 1997     

Effects of pool subcooling on boiling heat transfer in a vertical annulus with closed bottom

Effects of inlet subcooling on pool boiling heat transfer in a vertical annulus with closed bottom have been studied experimentally. For the test, a tube of 19.1 mm diameter and the water at atmospheric pressure have been used. Up to 50 K of pool subcooling has been tested and results of the annulus are compared with the data of a single unrestricted tube. The increase in pool subcooling results in much change in heat transfer coefficients. As the heat flux increases and the subcooling decreases, a deterioration of heat transfer coefficients is observed. The governing mechanisms are suggested as single-phase heat transfer and liquid agitation for the single tube while liquid agitation and bubble coalescence are the major factors at the bottom closed annulus.