Heat conduction with external cooling of gas-turbine rotor blades

With relevance to the study of external cooling of the working blades of gas turbines by air-liquid mixtures, a solution has been obtained for the problem of heat conduction of an infinite plate with periodic variations of the heat transfer coefficient and of the temperature of the surrounding medium.     

结霜工况下空温式翅片管气化器传热试验研究

为深入掌握低温液体在空温式翅片管汽化器内的气化情况以及其与翅片管表面霜层生长的相互影响规律,以液氮为介质进行了低温液体在空温式翅片管气化器内的气化试验。通过热电偶和刻度带分别对翅片管上不同位置的温度和霜层厚度进行了测量,并分析了翅片管表面霜层的生长规律及翅片管内低温液体的流动特性。结果表明:气化器表面结霜过程受冷表面温度影响较大,冷表面温度越低,结霜速率越大,霜层越厚。结霜工况下的气化器工作状态分预冷和稳态两种工作状态。预冷工作状态低温液体进入气化器后迅速气化,其过程包含气液两相和单气相两个换热段。稳态工作状态低温液体在气化器内气化经历单液相、气液两相、单气相三个换热段,单液相段翅片管表面结霜最为严重,单气相段翅片管表面无霜晶形成。因此认为,可通过分状态分段设计空温式翅片管气化器从而减弱结霜对翅片管传热的影响,提高气化器换热效率。     

滴形管换热性能的实验研究

采用特殊形状和表面的管子是最为常用、有效的强化换热手段。本文基于滴形管换热器回收天然气锅炉排烟余热,提出了烟气侧的换热系数实验关联式。通过改变换热管间的排列间距,在不同烟气流量下,对圆管和滴形管的换热性能及影响因素进行了分析。与实验数据比较,验证了实验关联式可正确反映凝结换热的特性。结果表明:不同烟气量通过滴形换热管的压损小于圆管,约为圆管的0.33~0.38倍;烟气温度降大于圆管;冷却水通过滴形管的温升高于圆管;换热系数滴形管比圆管的提高约7%,表明滴形管的换热性能优于圆管。因此对于有凝结换热过程发生时,滴形换热管具有强化换热的作用。     

Heat transfer in unsteady flow through a porous medium between two infinite parallel plates in relative motion

An approximate solution to the heat transfer in a flow of a viscous incompressible fluid through a porous medium bounded by two infinite parallel plates, the lower one stationary and the upper one oscillating in its own plane, is presented. Expressions for the mean temperature, the amplitude, and phase of the first and second harmonic of the rate of heat transfer and the mean rate of heat transfer are derived. The mean temperature is shown on graphs and the numerical values of the amplitudes and the phase are entered in a table. It is observed that the mean rate of heat transfer decreases with more ease of percolation but increases with increasing the frequency ω.     

Heat Transfer between a Gas-Droplet Medium and a High-Temperature Surface

A one-dimensional problem is treated, which is associated with heat conduction under conditions of contact between a finely divided gas-droplet medium with a highly heated surface when the temperature of the gas phase exceeds significantly the temperature of the liquid phase. Criterional equations are obtained for the calculation of the coefficient of heat transfer from a surface to a gas-droplet medium. Estimates are made of the time of complete evaporation of droplets and of the thickness of vapor-gas layer in the vicinity of the surface. The obtained results may be used to investigate the processes of cooling a metal.     

Solution of a generalized problem of heat or mass transfer in a bed

The kinetics of unrelated heat and mass transfer processes are examined for the case of motion of a liquid or a gas through a bed. The solution obtained applies to particles of various shapes (infinite plate, infinite cylinder, sphere).     

Modeling of rolled-sheet cooling

Stationary radiative and convective heat transfer on an infinite moving plate is studied numerically. A conjugate formulation of the problem allows one to correctly take into account the interaction between temperature fields of the plate and the surrounding gas medium. The determining influence of radiation on the formation of plate temperature is established. The effect of different regime factors on heat transfer is analyzed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 2, pp. 303–307, March–April, 1997.     

分离式热管管排组合与传热

分离式热管在不改变蒸发段和冷凝段总换热面积的情况下通过管排组合可以改变热管换热器局部热管的面积比来改变传热。研究分离式热管换热器在不同管排组合下传热差异,总结大温差传热下采用何种管排组合取得废热回收过程中的热管安全、高压发生器中均匀传热以及传热能力的均衡。     

穿孔翅片管的实验研究及翅片效率分析

在平翅片上开小圆孔是一种简单的强化翅片表面换热的措施,本文通过正交实验,研究了开孔大小和位置对换热性能和阻力性能的影响,得出了关联式。通过与普通平翅片管对比表明,最优穿孔型翅片管的换热系数可提高１２．５％,而阻力增加不超过７％,在同样的传热量的温差下,最优穿 形翅片管可以节约１０％的金属耗量,数值计算表明,最优穿孔形翅片的翅片效率比普通平翅片约低２％。     

An experimental study of the fluid flow and heat transfer characteristics in micro-condensers with slug-bubbly flow

Experiments were conducted to study the condensation flow pattern in silicon micro-condensers using water as the medium. Slug-bubbly flow was found to be one of the dominant flows in the micro-condenser and it was a major factor in determining the heat transfer and pressure drop properties of the fluid inside the micro-condenser. The transition from the slug-bubbly flow to a mixed flow pattern was studied. A correlation was obtained to predict when the transition of the flow pattern would occur. Only slug-bubbly flow existed under low steam mass flow rate and high heat transfer rate conditions. As the steam mass flow rate increased or the heat transfer rate dropped, the mixed flow pattern would then appear. In the slug-bubbly flow regime, the heat transfer coefficient and pressure drop in the micro-condensers were investigated in detail. It was found that micro-condensers with smaller channels could exhibit higher heat transfer coefficients with the same Reynolds number. The condensation heat transfer coefficient was higher than that in the tubes with the diameter of centimeter. Pressure drops in the micro-condensers with smaller channels were higher due to the increased transition loss. At the same time, the pressure drop in the micro-condenser was found to be lower than what could be predicted using the macro-scale correlation. Increasing the heat flux would create a longer bubble–film region and fewer unit cells in the micro-condenser resulting in an increased heat transfer coefficient and a decreased pressure drop.     

滴形管外含不凝气体自然对流凝结换热性能研究

本文在自然对流情况下,基于双膜理论和边界层理论,考虑气液界面热阻,建立了滴形管外气液膜厚度及传热系数的数学模型,得到不同初始参数下气液膜厚度、气液膜热阻、气液界面热阻、凝液量和传热系数沿管壁的分布规律。结果表明:其他条件不变,随着混合气压力的增大(由81 325 Pa增至121 325 Pa),液膜厚度增大约7%,传热系数减小约30%。随着不凝气体质量分数的增加(由0.1%增至10%),气膜厚度减小约52%,凝液量减少约85%,传热系数减少约82%。虽然气膜厚度减小,但气膜内不凝气体质量分数增加约58%,气膜热阻增加约61%。对于当量直径相同的滴形管,其他条件不变,滴形管下半部分曲率越大,越易发生液膜分离,传热系数越大。     

Characterisation and optimisation of flexible transfer lines for liquid helium. Part I: Experimental results

The transfer of liquid helium (LHe) into mobile dewars or transport vessels is a common and unavoidable process at LHe decant stations. During this transfer reasonable amounts of LHe evaporate due to heat leak and pressure drop. Thus generated helium gas needs to be collected and reliquefied which requires a huge amount of electrical energy. Therefore, the design of transfer lines used at LHe decant stations has been optimised to establish a LHe transfer with minor evaporation losses which increases the overall efficiency and capacity of LHe decant stations. This paper presents the experimental results achieved during the thermohydraulic optimisation of a flexible LHe transfer line. An extensive measurement campaign with a set of dedicated transfer lines equipped with pressure and temperature sensors led to unique experimental data of this specific transfer process. The experimental results cover the heat leak, the pressure drop, the transfer rate, the outlet quality, and the cool-down and warm-up behaviour of the examined transfer lines. Based on the obtained results the design of the considered flexible transfer line has been optimised, featuring reduced heat leak and pressure drop.     

Approximate method for calculating turbulent two-phase flow in a channel with permeable walls

Based on a model of a double-velocity and two-temperature medium the authors constructed a system of equations that describes plane or axisymmetric turbulent flow of a gas suspension in a channel with permeable walls. The system of equations of motion and heat transfer reduces to a system of ordinary differential equations, whose integration is much less difficult than solution of the initial system. The authors obtained the distribution of the velocity and local characteristics of turbulence in the channel with injection with allowance for the inverse effect of a condensed phase.     

Two-phase pressure drop and flow boiling heat transfer in an enhanced dimpled tube with a solid round rod insert

An experimental study was conducted on a 19.05 mm (outer diameter) dimpled enhanced tube to evaluate the in-tube two phase heat transfer and pressure drop performance in an annular section created between the enhanced tube and a solid round PVC rod. The purpose of the study was to understand the effect of forced early transition to annular flow on the pressure drop and heat transfer coefficient in a horizontal tube. The refrigerant studied was R-134a at a saturation temperature of 5 °C, heat flux range 2.5 to 15 kW m⁻², mass flux from 80 to 200 kg m⁻² s⁻¹ and inlet vapor quality of 0.12 to 0.72. Flow pattern and pressure drop results were obtained under adiabatic conditions. Under similar operating conditions the enhanced tube with a rod exhibited three times higher heat transfer performance versus same size smooth empty tube with lower pressure drop penalty at lower mas flux.     

LNG绕管式换热器壳侧单相传热模型的优化

天然气液化工艺中绕管式换热器的壳侧热力计算是当前亟待解决的问题之一,针对低温工况下壳侧传热模型的研究尚不多见,需要选取出适用的传热模型准确计算传热系数,为天然气液化工艺中绕管式换热器的设计选型和热力校核提供依据。本文比较分析了现有壳侧单相传热模型的优缺点,结合绕管式换热器壳侧低温实验数据,筛选出了适用于天然气液化预冷段的壳侧传热模型,并进行了优化。结果表明:对于天然气液化预冷段的壳侧传热系数计算,Abadzic传热模型计算精度最高、偏差范围最小、适用性最佳;Abadzic传热模型粘度修正后计算精度提高约50%,天然气液化预冷段的粘度修正系数可估算为1.05。     

Structure of pulse dispersed jet upon change in its frequency characteristics

Intensities of heat and mass transfer for continuous and pulse spray in interaction with the vertical surface of a heat exchanger differ considerably from one another when the time-averaged spraying rate [1–6] is kept constant. This difference is determined by characteristic properties of gas-drop flows of continuous and pulse sprays and their interaction with the heat-exchanging surface. Results of experimental studies of basic hydrodynamic parameters of pulse drop flow are presented: velocity and drop size dispersions, spectral characteristics of the kinetic energy of the spray at various distances from the source, and influence of the gas concurrent flow on these parameters. A difference in the heat transfer is shown upon variation of the pulse drop flow parameters from the heat transfer at the stationary supply of spray.     

微胶囊相变乳状液在液冷服中的应用前景

介绍了一种能替代传统蓄冷介质的潜热型功能性热流体——微胶囊相变乳状液，它具有蓄冷密度大、传热性强、无过冷现象、价格低廉等优点。在传热量一定的条件下，微胶囊相变乳状液作为传热介质比水所需流量少，流动时摩擦阻力小，若用它作液冷服的冷却液可以大大减小换热管路的尺寸，降低循环泵的功耗。更重要的是，材料在发生相变前后其自身温度基本保持不变，解决了目前大多数液冷服穿着不舒适的问题。     

Condensation heat transfer and two-phase frictional pressure drop in a single minichannel with R1234ze(E) and other refrigerants

R1234ze(E), trans-1, 3, 3, 3-tetrafluoropropene, is a fluorinated propene isomer which may be a substitute of R134a for refrigeration applications. R1234ze(E) has a much lower GWP_100-years than that of R134a. In this paper, the local heat transfer coefficient during condensation of R1234ze(E) is investigated in a single minichannel, horizontally arranged, with hydraulic diameter equal to 0.96 mm. Since the saturation temperature drop directly affects the heat transfer rate, the pressure drop during adiabatic two phase flow of R1234ze(E) is also measured. Predictive models are assessed both for condensation heat transfer and pressure drop. A comparative analysis is carried out among several fluids (R1234ze(E), R32, R134a and R1234yf) starting from experimental data collected at the same conditions and using the Performance Evaluation Criteria (PEC) named Penalty Factor (PF) and Total Temperature Penalization (TTP) to rank the tested refrigerants in forced convective condensation.     

Reasons for drop in shell-and-tube condenser performance when replacing R22 with zeotropic mixtures. Part 2: investigation of mass transfer resistance effects

Detailed calculations of condensation outside a column of horizontal smooth tubes have been carried out in order to investigate the influence of mass transfer resistance for a zeotropic refrigerant mixture. Diffusive transport is calculated locally in both phases. Calculation results show that mass transfer resistance in the gas phase reduces heat transfer by 10–20% for a binary mixture with a glide similar to that of R407C.The decrease in heat transfer due to poor mixing in the condensate is from 15 to 65% for the conditions investigated; the lower the duty, the greater the decrease. Results show that if assuming mixing in the condensate by diffusion only, the degrees of mixing and the duty dependency are similar to results in earlier work where calculations were matched to experimental data. Diffusion is likely to be the dominant mixing mechanism in the condensate, and the mixing is poor under certain conditions, which might explain the drop in condenser performance for some heat pump applications.     

Mathematical modelling of dropwise condensation on textured surfaces

Vapor-to-liquid phase change in the form of discrete drops on or underneath a substrate is called dropwise condensation. The process is hierarchical in the sense that it occurs over a wide range of length and timescales. As the associated heat transfer coefficient is much higher than the film and mixed mode of condensation, it is of considerable interest in applications. The present study is focused on mathematical modelling of dropwise condensation process at multiple scales. The model includes formation of drops at the atomistic scale, droplet growth, coalescence, instability, slide off and fall-off, followed by fresh nucleation of liquid droplets. The model shows that the largest stable cluster size in the atomic model matches the minimum drop radius estimated from thermodynamic considerations. The minimum drop radius is insensitive to surface texturing and does not provide controllability at larger length and timescales. A closer examination of droplet distribution over the substrate reveals that small drops are locations of high heat transfer rates, which diminishes with increasing drop radius. The largest drop diameter depends on its stability and hence, the interfacial forces at phase boundaries. Therefore, drop instability controls the heat transfer coefficient in dropwise condensation. Enhancement of heat transfer necessitates that these drops grow with time, become unstable and be swept away as quickly as possible. Enhancement may be achieved either by (i) inclining the substrate or (ii) by creating an interfacial force at the three-phase contact line by a wettability gradient over the horizontal substrate, inducing drop motion. Wall heat transfer and shear stress under moving drops have been determined using a CFD model. A simple model of coalescence has been adopted in this work. Simulation studies on the effect of fluid properties, surface inclination and its wettability condition on drop size distribution, cycle time, heat transfer coefficient, and wall shear stress are comprehensively discussed in the present article.