Heat/mass transfer from a drop translating in steady and time-periodic electric fields: External problem

Enhancement of heat or mass transport from a spherical drop of a dielectric fluid translating in another dielectric fluid in the presence of steady and time periodic electric fields (both uniform and non-uniform) is investigated in this paper. The external problem or the limit of the majority of the transport resistance being in the continuous phase is considered. Using a finite volume formulation, the transient energy (species) conservation equation is solved for Peclet numbers (Pe) varying from 10 to 1000 and dimensionless electric field frequency (ω¹) from 10 to 50,000 using a fully implicit method. To map the infinite domain in the radial direction, an exponential transformation is employed that provides a fine grid spacing near the drop surface where sharp variations are expected and a coarser grid in the far field where low gradients prevail. The transient temperature distribution and the local and the average Nusselt numbers are obtained and the heat/mass transfer enhancement due to the application of electric field is determined. The effect of electric field is expressed in terms of L, the ratio of the maximum electric-field-induced surface velocity to the translation-induced surface velocity. For the steady and time-periodic electric fields, the heat transfer enhancement increases monotonically with Pe and with L. Also, the heat transport rate is higher when the continuous phase is more viscous compared to the dispersed phase. The steady uniform electric field gives the highest average Nusselt number for heat transfer from the drop surface to the continuous phase, followed by the non-uniform time periodic electric field and then the time periodic uniform electric field. The enhancement relative to pure translation exhibits a non-monotonic dependence on electric field frequency but the highest value is obtained at ω¹ = 0. Earlier studies have shown that there is significant enhancement in the heat/mass transfer in the drop interior with a time periodic electric field compared to a steady uniform electric field when the majority of the transport resistance is in the drop. The results presented here show that an opposite behavior is obtained in the drop exterior, i.e. a steady electric field provides higher heat/mass transfer enhancement compared to a time periodic electric field, when the bulk of the transport resistance is in the continuous phase. Therefore whether the steady or the time periodic electric field provides the most enhancement of heat/mass transfer for a conjugate problem will depend on the relative transport resistance in the two phases.     

Transient heat and mass transfer in a drop experiencing absorption with internal circulation

Transient heat and mass transfer associated with a moving liquid during absorption was numerically studied in this work. The roles played by the internal circulation inside the droplet and the exorthermic heat effect were demonstrated. The numerical results reveal that the significant absorption enhancement by internal circulation becomes neglible with the increase of exothermic absorption heat. The highly exotermic system of , which is used as a typical refrigerant/absorbent combination in commercial absorption heat pump (AHP), was selected as an example to illustrate this point.     

Study on enhancement of condensation heat transfer using an electric field

Enhancement of condensation heat transfer using a nonuniform electric field was experimentally investigated for horizontal smooth and low‐finned tubes. In the experiments, a wire electrode parallel to the tube was placed beneath the tube. The experimental parameters were the distance and voltage between the wire electrode and the tube, and the condensation heat flux. Results of the present experiment for the low‐finned tube indicate that, as the applied voltage increases, the enhancement ratio increases steeply at a certain voltage and it reaches 2.4. It was observed that the condensate flow pattern falling down from the bottom of the tube changed from a flat film to circular columns at a critical voltage. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 269–279, 2000     

A magnetic vortex generator for simultaneous heat transfer enhancement and pressure drop reduction in a mini channel

An active vortex generator is proposed for heat transfer enhancement in heat sinks and heat exchangers and removal of highly concentrated heat fluxes. It is based on applying a uniform magnetic field of permanent magnets to a magnetic fluid (ferrofluid) flowing in a heated channel. Numerical simulations are carried out for a 2 Vol% ferrofluid at different Reynolds numbers (150‐210) and magnetic field intensities (0‐1400 G) to investigate the possibility of simultaneous heat transfer enhancement and pressure drop reduction by the proposed method. Comparisons are also made with the other conventional vortex generators. Results indicate that the external magnetic field acts as a vortex generator that changes the velocity distribution, improves the flow mixing, and thereby increases the convective heat transfer. Surprisingly, the heat transfer enhancement is accompanied by a decrease of the friction coefficient due to the flow separation and decrease of the flow contact with the surface. It is also concluded that increasing the magnetic field intensity, decreasing the flow rate, and adding a second identical magnetic vortex generator have favorable effects on both pressure drop and heat transfer. A maximum of 37.8% enhancement of heat transfer with a 29.18% reduction of pressure drop has been achieved at the optimum condition.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.     

Conjugate mass transfer to a spherical drop accompanied by a second-order chemical reaction inside the drop

Conjugate mass transfer between a drop and a surrounding fluid flow with second-order (inclusive the particular case - pseudo-first-order), irreversible chemical reaction in the dispersed phase has been analyzed. The dispersed phase reactant is insoluble in the continuous phase and its complete depletion is allowed. Two sphere models were considered: the rigid sphere and the fluid sphere with internal circulation. For each sphere model two hydrodynamic regimes were employed: creeping flow and moderate Re numbers. Slow and fast chemical reactions were analyzed. A single, constant value was considered for Pe, Pe=100. The influence of the diffusivity ratio on the particle average concentrations, total mass transferred and enhancement factor is studied. The values obtained for the enhancement factor of the pseudo-first-order chemical reaction are compared with solutions provided by published predictive equations. The chemical reaction enhances the mass transfer rate even for values of the modified Hatta modulus smaller or considerably smaller than one. For the flow patterns and sphere models considered in this work, the enhancement factor is independent on hydrodynamics.     

Experimental study of the effect of winglet location on heat transfer enhancement and pressure drop in fin-tube heat exchangers

Local heat transfer coefficients were measured on fin-tube heat exchanger with winglets using a single heater of 2 inch diameter and five different positions of winglet type vortex generators. The measurements were made at Reynolds number about 2250. Flow losses were determined by measuring the static pressure drop in the system. Results showed a substantial increase in the heat transfer with winglet type vortex generators. It has been observed that average Nusselt number increases by about 46% while the local heat transfer coefficient improves by several times as compared to plain fin-tube heat exchanger. The maximum improvement is observed in the re-circulation zone. The best location of the winglets was with ΔX = 0.5D and ΔY = 0.5D. The increase in pressure drop for the existing situation was of the order of 18%.     

Internal heat generation in a discrete heat source: Conjugate heat transfer analysis

In the present work, we conduct an asymptotic and numerical analysis for the cooling process of a discrete heat source, which is placed in a rectangular-channel laminar cooling flow. In our physical model, the heated strip is embedded in a substrate, generating continuously a uniform volumetric heat rate. We assume that this heat-generation mechanism is due to an electrical current in the heat source. Hence, heat losses to the cooling fluid and to the substrate material during this process are presented. The governing equations of the cooling flow and the participating solid are reduced to an integro-differential equation that predicts the temperature variations of the heat source. We show that the conjugate heat transfer process is controlled by a conjugate nondimensional parameter, here denoted by α, which determines the basic heat transfer regimes between the cooling flow and the discrete heat source.     

Effect of coil-wire insert on heat transfer enhancement and pressure drop of the horizontal concentric tubes

In the present study, the heat transfer characteristics and the pressure drop of the horizontal double pipe with coil-wire insert are investigated. The inner and outer diameters of the inner tube are 8.92 and 9.52 mm, respectively. The coiled wire is fabricated by bending a 1 mm diameter of the iron wire into a coil with a coil diameter of 7.80 mm. Cold and hot water are used as working fluids in the shell side and tube side, respectively. The test runs are performed at the cold and hot water mass flow rates ranging between 0.01 and 0.07 kg/s, and between 0.04 and 0.08 kg/s, respectively. The inlet cold and hot water temperatures are between 15 and 20 °C, and between 40 and 45 °C, respectively. The effect of the coil pitch and relevant parameters on heat transfer characteristics and pressure drop are considered. Coil-wire insert has significant effect on the enhancement of heat transfer especially on laminar flow region. Non-isothermal correlations for the heat transfer coefficient and friction factor are proposed. There is reasonable agreement between the measured data and predicted results.     

Performance evaluation of flattened tube in boiling heat transfer enhancement and its effect on pressure drop

An experimental investigation has been carried out to study heat transfer and pressure drop characteristics of R-134a flow boiling inside a horizontal plain tube and different flattened tubes. Round copper tubes with an inner diameter of 8.7 mm were flattened into an oblong shape with an internal height of 6.6 mm, 5.5 mm, 3.8 mm, and 2.8 mm. The test apparatus was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Analysis of the collected data showed that, by flattening the round tube, the heat transfer coefficient and pressure drop increased simultaneously. The performance of these tubes from the point of view of heat transfer enhancement and pressure drop increasing were evaluated. It was concluded that, the tube with an internal height of 5.5 mm has the best performance compared with the other flattened tubes. Finally, based on the present experimental pressure drop data, a correlation was developed to estimate the pressure drop in flattened tubes. This correlation predicts the experimental data of the present study within an error band of ± 20%.     

在第二类吸收式热泵中对螺旋槽管的换热性能研究

为了提高在以溴化锂为工质的第二类吸收式热泵吸收器的性能,在第二类吸收式热泵吸收器内对不锈钢螺旋槽管,即不锈钢光滑管的传热传质性能进行了实验研究.发现螺旋槽管的传热传质性能约为光滑管3倍,螺旋槽管内热媒工质-水的流体阻力系数是光滑管的17～20倍;应用于第二类吸收式热泵中间大大降低换热面积,促进热泵的高效紧凑化.     

Numerical investigation of pressure drop reduction without surrendering heat transfer enhancement in partially porous channel

The present study is to investigate the numerical simulation of steady laminar forced convection in a partially porous channel, with four dissimilar porous-blocks, attached to the strip heat sources at the bottom wall. The analysis is based on the Navier–Stokes equation in the fluid field, the Darcy–Brinkman–Forchheimer flow model in the porous field, and the energy equations for two thermal fields. The effects of variations of different parameters such as porous blocks Darcy numbers, arrangements of dissimilar blocks, Forchheimer coefficient, Reynolds number, thermal conductivity and Prandtl number are investigated and the velocity and temperature fields are presented and discussed. In the dissimilar partially porous channel, it is found that when the blocks sorted from the lowest to the highest Da in the flow direction, the total heat transfer enhancement is almost the same as in the similar porous channel (Nu/Nu_sim = 92%), while the total pressure drop is considerably lower (P/P_sim = 28%). In addition, reverse arrangement of porous blocks is suggested to prepare more uniform temperature gradient in all heat sources.     

Experimental study of the flow boiling heat transfer enhancement and pressure drop due to the bubble behavior restricted by a screen sheet

A unique method previously proposed by the authors was applied to the heat transfer augmentation in the flow boiling field. In this method a screen sheet was placed on the horizontal heated surface where bubble nucleation occurred. Generated vapor bubbles were trapped between the screen and the wall, became flat, and moved along the surface. This restricted bubble behavior caused the heat transfer enhancement. Three types of screen sheet were tested in the present experiment and the effect of the screen on the heat transfer and two‐phase flow characteristics was investigated. In two of these cases, the screen was displaced upward by the bubble nucleation. Compared with the ordinary flow boiling case, heat transfer was enhanced by a factor of 1.2 to 6 within the present experimental range. Using a simple flow model, it was made clear that the effect of the height of the displaced screen was important in evaluating the increase in pressure drop. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 319–329, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10094     

Pressure drop and point mass transfer in a rectangular microchannel

This paper describes the pressure drop and local mass transfer in a rectangular microchannel having a width of 3.70 mm, height of 0.107 mm and length of 35 mm. The pressure measurements were carried out with distilled water as working fluid at Reynolds numbers in the range of 100–845, while mass transfer measurements with a chemical solution at Reynolds numbers in the range of 18–552 by using the electrochemical limiting diffusion current technique (ELDCT). Experimental friction factors were slightly higher than those calculated by theoretical correlation. The Sherwood number correlation was also obtained.     

Experimental and Numerical Investigation of Enhancement of Heat and Mass Transfer in Adsorbent Beds

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多效管式太阳能苦咸水淡化装置热质传递强化研究

针对管式太阳能蒸馏苦咸水淡化装置产水率低的问题,提出了一种多效管式太阳能苦咸水淡化装置。文章介绍了淡化装置的构造和运行原理,提出了两种强化管式太阳能苦咸水淡化装置内部传热传质效能的方法,测试了装置在定功率和定温度运行条件下蒸发温度、冷凝温度和产水量的变化,研究了对其第二效套筒风冷和水冷条件下,装置淡水产量随运行温度变化的规律。研究结果表明:在输入功率为335 W时,装置的性能系数达到1.37;当加热温度为80℃,对装置套筒进行风冷强化时,淡水产量为1.248 kg/h,比无风冷时的装置淡水产量增加了82.5%;当对装置套筒进行水冷强化时,淡水产量为1.556 kg/h,是无水冷时的装置淡水产量的2.28倍。     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

Mass and heat transfer enhancement of chemical heat pumps

An inert additive, expanded graphite (EG), has been prepared and used to enhance the heat and mass transfer process of chemical heat pumps. The effects of mixing ratio and mixing method on the chemical reaction time are investigated.     

Experimental study of heat and mass transfer on an absorber with several enhancement tubes

An experimental study of the absorption process of water vapor into a lithium bromide solution was performed. For the purpose of the development of a high-performance absorption chiller/heater utilizing a lithium bromide solution as the working fluid, it is the most effective way to improve the performance of the absorber with the largest heat transfer area of the four heat exchangers. This paper considers a bare tube, bumping bare tube, floral tube, and twisted floral tube for the absorber of an absorption chiller/heater. The floral and twisted floral tubes have about 40% higher heat and mass transfer performance than the bare tube conventionally used in an absorber. Therefore, floral and twisted floral tubes are expected to realize high heat and mass transfer performance. © 1999 Scripta Technica, Heat Trans Asian Res, 28(8): 664–674, 1999