Heat Transfer Enhancement of Heat Exchangers

This lecture provides a general introduction to the subject of the Institute. Introductory remarks are made about heat transfer enhancements, surface augmentation or intensification in heat exchangers. The preview of lecture topics is introduced, indicating their contributions to the institute.

This review considers the many techniques that have been developed to enhance convective heat transfer. There is the desire to encourage or accommodate high heat fluxes. After introducing the techniques, the applications to the various modes of heat transfer (single-phase free convection, single-phase forced convection [including performance evaluation criteria, active techniques, and compound techniques], pool boiling, convective boiling/evaporation, vapor-space condensation, and convective condensation) are described. Comments are offered regarding the generations of heat transfer technology; advanced enhancement represents 3rd generation heat transfer technology.

The Lecture will review the growing concern of scientific and political community of the world has been expressed through the definition of sustainability as a parameter to measure our future behavior in the use of the available resources on our planet. Historically speaking, it was noticed in very early days of our past that human concern has been expressed in several documents which emphasize need for the rational use of global resources and the sustainability as a moral, cultural and philosophical pattern of our individual and collective behavior.Special attention is devoted to the defmition of sustainability and its generic meaning. In this respect a particular attention is given to the discussion of different aspects of sustainability in the present world. In order to present an engineering approach to the sustainable development, attention is focused to the review of sustainability criteria, as they have to be introduced in the future productsThe main emphasis is given to review a potential development in the energy engineering science that may lead to a sustainable energy development. Seven major areas are listed with specific problems and their relevance to the sustainable energy development. This includes the following areas: energy resources and development; efficiency assessment; clean air technologies; information technologies; new and renewable energy resources; environment capacity; mitigation of nuclear power treat to the environment.In order to quantify criteria for the sustainability assessment of any design of thermal system element indicators are defined to meet this requirement. In this respect, the efficiency of resources use and technology development is of fundamental importanceSpecial emphasize is devoted to the example related to the analysis of the current design method for the condenser design and its comparison with the respective sustainability criterion. In this respect the power plant condenser design is shown in the potential prospect for the new design criterion which might lead to the development of the new method for the condenser design or the better understanding for its potential improvement.

This discourse discusses the use of extended surface heat transfer in heat exchangers. The size and cost of an exchanger is dependent upon the heat duty, the allowable pressure drop, and the geometry of the heat transfer area. Often one heat transfer resistance within the overall heat transfer dominates and this determines the amount of area required for the duty. The dominance of this resistance is often broken by the use of extended surfaces (fins), which will result in a lower area and hence smaller exchanger. The extended surface performance can be predicted by a number of indicators: fin effectiveness, fin efficiency, enhancement and augmentation. This discourse will consider which, if any, of the above performance indicators are the best.

The in-tube evaporation heat transfer of HFC-134a was examined for a smooth tube and five mnicrofin tubes. The heat transfer coefficient in the smooth tube was compared with four correlations. The Yoshida correlation wvas found to be the best in annular flow, and the Kandlikar correlation (with F_K= 2.33) was the best in stratified flow, but the latter needs clarification for its applicability to the low quality range, because of the strong nucleate boiling effect cused by the large value of F_K. The effect of the spiral angle of microfin tubes was investigated to determine the optimal spiral angle. Spiral angles of 66. 12, 18. 25 and 44 degrees were tested at constant mean diameter, 8.71 mnm (0.343 in.) and number of fins. 60. The optimal spiral angle was found to be mainly dependent 0n the mmmaSs flux, and was 18 degrees for a mass flux of G=50 kg/m2 .s (36.865 lb_m/ft2.), 6 degrees for G=100 kg/m2 .s (73,730 lb_m/ft2.h), and 6 or 18 degrees for G=200 kg/m2.s (147,460 lb_m/ft2.h).

Wing-type longitudinal vortex generators (LVGs) can be used as fins or fin modifications for heat transfer enhancement in compact heat exchangers. For the last 15 years a group at the Ruhr-University Bochum has studied systematically the characteristics of heat transfer surfaces with vortex generators. Recent results are reported here. Three heat transfer enhancement modes may be induced by LVGs: developing boundary layer, swirl (longitudinal vortices) and flow destabilization. First these enhancement modes were studied numerically and experimentally for a base configuration consisting of a rectangular winglet array in channel flow at a Reynolds number of 175. Then finned oval tube heat exchanger elements with and without punched delta winglets were studied numerically. Winglet number and arrangement on the fin were varied in the Re range from 100 to 500. Flow structure, temperature distribution, local and global heat transfer, and flow loss penalty were presented. Staggered winglet arrangements were superior to in-line arrangements. The fin heat transfer surface may be reduced by more than 50% compared to a plane fin for identical heat duty, pressure loss, mass flow rate and temperature difference of the hot and cold medium. Direction for further research is indicated.

Wing-type longitudinal vortex generators were employed for performance enhancement of finned tube heat exchanger elements. The effect of finite fin heat conduction on the performance of fins and longitudinal vortex generators was considered. Three-dimensional developing laminar flows and conjugate heat transfer in high performance finned oval tube elements, with and without punched deltawinglets, were studied numerically with a Finite-Volume Method. Body-fitted grids were used to satisfy exactly the thermal and hydrodynamic boundary conditions. The conjugate heat transfer was realized by iterations of convection in the flow field and conduction in the fin. Reynolds numbers of 100<Re<500, and fin conduction parameters of 100<Fi<1000 were varied. Temperature fields, local fin heat transfers and fin efficiencies were presented. Performance of finned oval tube elements with up to four in-line or staggered winglets was evaluated. For the investigated base configuration, performance enhancement through winglets increased with increasing Reynolds number and fin conductivity. For performance comparisons of different configurations, results of isothermal fins could be transferred to non-isothermal fins. The performance enhancement by winglets for fins with finite conduction may be reduced by about 20% in practical applications compared to that for isothermal fins.

This lecture considers flow fields, friction factors, and local and average heat transfer coefficients in rib-roughened ducts for applications in compact heat exchangers and cooling systems in gas turbine systems. Details of the flow pattern and the influence of rib configuration, rib angle, rib pitch and height are discussed, and physical interpretations of the results are provided.

The present study focuses on the airside performance of fin-and-tube heat exchangers. Contents of this lecture include the data reduction method of the airside perforinance and the updated correlations for the typical fin-and-tube heat exchangers which including plain, wavy, louver, and convex-louver fin patterns. It is strongly recommended that the investigators should clearly address the details of their data reduction methods. In addition, appropriate ε-NTU relationships should be carefully examined before applying them to the determination of the heat transfer coefficients. It is also suggested that the entrance and exit loss should be included in the estimation of friction factors.

This lecture presents a rationally based methodology to model air-cooled condenser and water heating coils. The effect of complex circuiting can be taken into account by the proposed indexing technique. The calculated results agree well with experimental data. It is found that the mal-distribution of refrigerant flow is strongly related to thermal-hydraulic interactions between air and refrigerant. In addition to the model, preliminary test results of a 1-circuit wavy finned condenser having different arrangements are also reported.

This lecture focus attention on the transport of heat and momentum in plateand-frame heat exchangers. In particular numerical procedures are considered and comparisions with experimental data are provided. A brief review of previous investigations is also given.

Experimental and numerical investigations of the forced convection heat transfer in flat channels with rectangular cross section are presented in this paper. The heat transfer is enhanced by rib-roughened surfaces applied to the wider walls of the duct. The flow rates have been varied between the Reynolds numbers 500 and 10.000, covering the range from laminar to low turbulent flow. Various configurations (rib: shape, size, spacing, angle of attack, arrangement, duct: width, height, wall temperature) have been investigated. The local heat transfer coefficients have been obtained by holographic interferometry. Local hydrodynamic parameters (with special interest to turbulent kinetic energy and Reynolds shear stresses) were measured by a laser Doppler velocimeter (LDV). According to the configurations examined experimentally, numerical CFD-calculations also have been performed. To increase the accuracy of the numerical calculations for problems with simultaneous laminar and turbulent flow regimes, an empirical procedure is proposed.

Copper mesh, compressed and formed into porous matrices of various shapes and sizes, has been routinely used in high heat load/flux component cooling at the Advanced Photon Source (APS) to significantly enhance the heat transfer performance. Substantial research has been performed at the APS over the last eight years in order to better quantify and optimize the selection of the mesh copper matrix attributes (porosity, wire size, core size, bonding technique, etc.) for various water-cooled component applications in single phase. The same mesh configuration can also be applied in cryogenic cooling of optical components, such as monochromators, mirrors and multilayers, with liquid nitrogen. This paper reviews the experimental data and the analytical calculations, compares the data with existing single- and two-phase correllations, and interprets the results for a cryogenically cooled monochromator.

This paper describes a series of studies to understand the mechanism of boiling in structured surfaces having sub-surface tunnels, and surface pores. Innovative visualization that allowed observation within the tunnels conclusively shows that in saturated boiling, the tunnel is vapor filled, except for thin liquid films on the tunnel walls and menisci in the corners. Evaporation on liquid menisci in the tunnel corners is the principal boiling mechanism for the structured surfaces. Experiments were performed to define the effect of pore diameter, pore pitch, and tunnel size on performance in structured boiling surfaces. The Dry-Out Heat Flux increases with the increase of total open area. At a certain reduced heat flux, part of the tunnel will become flooded and the performance will be reduced. Smaller pore size will inhibit flooding at reduced heat flux. Visualization experiments were performed to determine the bubble departure diameter, bubble frequency, waiting and growth periods, and nucleation site density. These data provided the basis for models to predict these parameters as a function of surface geometry and fluid properties. A mechanistically based model was developed to predict the boiling heat flux as a function of (T_w — T_s), pore and tunnel dimensions, and fluid properties. The model predicted the heat transfer data for R-11, R-123, R-134a, and R-22 within ±3 3%(0.20 MSD) .

During the past fifteen years heat transfer in oscillatory flows has become the subject of increasing interest in the engineering community. Applications of oscillatory flows include, for example, the cooling of electronic equipment or alternative, environmentally safe refrigeration technologies, such as thermoacoustic refrigeration, pulse tubes or Stirling refrigerators. Important components of these refrigerators are their heat exchangers. In such devices the working fluid is subjected to oscillatory forcing which is a key part of the process, as opposed to situations where oscillations are generated with the aim to enhance heat transfer. Heat transfer in oscillatory, and often compressible, flows has not yet been completely understood, and the lack of design methodologies for heat exchangers in such flows is one reason that efficiencies of these devices are limited. In this paper, convective heat transfer in steady flow is contrasted to measurements of heat transfer in oscillatory flow obtained by holographic interferometry in operating regimes characteristic of the heat exchangers. The results of the research are expected to lead to guidelines that will allow the design of heat exchangers with improved heat transfer performance.

A ceramic heat exchanger using a fluidized bed for generating high temperature gas was developed for the coal-fired gas turbine combined cycle. Smooth and externally finned ceramic tubes were used for the heat transfer tubes. The characteristics of heat transfer coefficients and the performance of the heat exchanger were evaluated. It was certified that the heat transfer coefficient on the outside wall of the tube was sharply increased by using both a fluidized bed and the finned heat transfer tubes. But, the overall coefficient of heat transmission was dominated by the heat transfer inside the tube . Therefore, heat transfer enhancement on the inside wall of the tube is needed. This was accomplished by external finning. It was found that relaminarization occurred at high heating rates. Experiments were performed and heat transfer coefficients on the inside wall of the tube at high heating rates were evaluated.

Thin liquid films are utilized as an important component in various heat transfer processes because of their high heat transfer rate at low feed rates and with small temperature difference. Evaporators employing such characteristics are widely found in refrigeration systems, natural gas and air liquefying facilities, petrochemical and distillation plants, and more recently are proposed for use in OTEC systems. This paper reviews recent developments on evaporation and boiling heat transfer to falling films on horizontal tubes and their enhancement. Main items focused here are; (1) analysis and experiment for evaporative heat transfer to falling films, (2) empirical formulas for predicting heat transfer coefficient, (3) falling film breakdown and resulting deterioration of heat transfer, (4) analysis of enhanced heat transfer in grooved surface, (5) experimental results for heat transfer enhancement on grooved tubes, and (6) enhanced boiling heat transfer in falling films on grooved tubes.

In this study, the incompressible turbulent flow in a circular channel with a series of conical hollow inserts, placed axisymmetrically, is investigated numerically and experimentally. In computations, FLUENT, a commercially available CFD code, is used. The inserts are installed in a diverging structure towards the incoming flow. The effect of the inserts geometry on the heat transfer at the wall is evaluated for a range of Reynolds number between 3000 and 20000. It is demonstrated that a significant enhancement in heat transfer from the tube wall occurs in comparison with the conventional plain tube, and the level of the enhancement depends on the flow rate, the type of the inserts and the installation positions of the inserts. In order to see the flow fields, the velocity profiles are shown. Using the entropy minimization technique the optimum geometries and the installation distances were estimated. The results obtained through the numerical simulation are compared with the experimental ones and, a satisfactory agreement is found.

Transport processes — heat and mass — play an important role in energy conservation. To optimise these processes, optical measuring techniques can be of great help. An overview is given on various optical measuring techniques, which give insight into transport processes, especially with heat transfer and with combustion. A short introduction into holography, holographic interferometry as well as Rayleigh Scattering and Laser Induced Fluorescence show, together with examples from several fields of thermo-fluiddynamic research work, the capabilities of optical measuring techniques to improve transport processes.

The principal objective of this study is the analysis and improvement of the combined heat and mass transfer of various newly designed rotary solid storage elements.Rotary exchangers usually consist of a combination of a carrying material and an adsorbing storage element. The heat and mass transfer surface is of cellular structure usually referred to as matrix. Using the holographic interferometry experiments were conducted in order to determine the most effective duct geometry for advanced heat and mass transfer. Therefore various duct geometries like sinusoidal, rectangular, triangular and semicircular ducts were investigated at different temperatures and flow velocities. According to the configurations that were examined experimentally a numerical calculation has been performed.Rotary exchangers usually consist of a combination of a carrying material and an adsorbing storage element. The heat and mass transfer surface is of cellular structure usually referred to as matrix. Using the holographic interferometry experiments were conducted in order to determine the most effective duct geometry for advanced heat and mass transfer. Therefore various duct geometries like sinusoidal, rectangular, triangular and semicircular ducts were investigated at different temperatures and flow velocities. According to the configurations that were examined experimentally a numerical calculation has been performed.

The propagation of gaseous explosions is governed by the interaction of chemical kinetics with the molecular and turbulent heat and mass transport. Combustion processes like deflagration and detonation depend on the different valence of physical effects under certain conditions. Geometry and the expansion flow of the flame itself affect the turbulence and therefore the transport of fuel into the reaction zone. The present paper discusses the different hydrogen combustion processes and reports on the experimental investigations of transport phenomena during flame propagation with highly blocking obstacles. Several facilities have been operated with sophisticated optical measurement techniques like high speed schlieren videographie, laser induced predissociation fluorescence and laser doppler velocimetry to obtain detailed information about the combustion process. It will be shown that the turbulent quenching of flames leads to an amount of free radicals resulting in sensitive clouds of those radicals with corresponding high chemical reaction rates, which has a strong influence on the efficiency of the combustion processes.

This study deals with the drying of a dispersed varnish which is applied as a thin film on paper. Global measurements are performed in order to record the drying curves, whereas single phenomena are examined by local measurements. Their results are shown, and the basic equations for the calculation of the drying are presented.

Abstract. A large-bore, four-stroke engine, with high pressure injection and compression ignition, running with hydrogen, is a modern concept for clean energy conversion without CO₂ emission. The key processes for reliable engine operation are a good mixture formation, a reliable ignition and efficient combustion. The investigations of these processes are carried out in a rapid compression machine, with modern optical measurement techniques.

The installation of adiabatic horizontal partition plates between fins at constant intervals has been investigated. Both the partition plate height and the pitch values, the parameters used for the computation, affect the magnitude of the local heat transfer coefficient. Exceeding certain values for the height of the partition plates and the pitches, transition to turbulence has been observed. Since the plates are adiabatic they did not work as extended heat transfer surfaces but as promoters. Thus, the promoters redirecting the flow can be used to enhance the heat transfer in both laminar and turbulent cases. In this study, the flow and temperature fields around the plates have been computed numerically and a correlation between the partition plate height, the pitch and the heat transfer performance has been obtained.

The effect of different heater surface configurations on two-phase flow instabilities has been investigated in single channel, forced convection , up-flow and horizontal systems. Freon-11 is used as the test fluid, and different heater tubes with various surface configurations have been tested at different heat inputs. The effect of augmentation on flow instabilities is studied with three different tubes made of different inner surfaces, at various inlet temperatures. All experiments are carried out at constant system pressure and exit restriction. Steady-state characteristics of the system composed of different tubes are found and presented in pressure-drop versus mass flow rate diagrams. Dynamic instabilities, such as pressure-drop-type, density-wave-type instabilities and thermal oscillations are found to occur in every system, and boundaries for the appearance of these oscillations are found. The dependence of the characteristics of these oscillations on the augmented surfaces is emphasized, and comparisons between the bare tube and the augmented tubes are made.

Microfin tubes have become the most important type of heat transfer augmentation for intube evaporation of refrigerants in direct-expansion evaporators, climate-control systems and heat pumps. These tubes are characterized by a large number of small fins inside small diameter tubes, providing heat transfer augmentations in the range from 1.5 to 4 times with only modest two-phase pressure drop penalties. The most recent experimental research on evaporation of pure fluids in microfin tubes is reviewed and the latest prediction methods are critically assessed.

Research on heat transfer to refrigerants has gained importance in the 1990’s in response to the massive conversion of the refrigeration and air-conditioning industry from refrigerants ‘retired’ by the Montreal protocol (R-11, R-12, R-502, etc.) to more environmentally-safe refrigerants (R-134a, R-123, R-407C, R-410A, ammonia, hydrocarbons, etc.). Lubricating oil from these systems’ compressors, absorbed by the refrigerant charge circulating in these systems, can have a dramatic effect on boiling performance and represents one of the old, unresolved problems of refrigeration heat transfer. The present survey presents a summary of the recent research on evaporation of refrigerant-oil mixtures inside plain and microfin tubes, including experimental studies and new prediction models.

This paper presents an experimental study dealing with basic nucleate boiling from a finned surface placed in a narrow channel. The influence of the channel width, the orientation of the base surface (horizontal or vertical), and the pressure are discussed. The experiments were performed in a saturated pool of FC-72, while the channel widths were varied from 40 mm up to 0.5 mm. The experimental data are compared with those obtained in the case of the unconfined extended surface. Channel width reduction does not affect the heat transferred to the liquid in the case of vertical orientation of the base surface, but it causes a drastic reduction in the heat transfer behavior in the case of a horizontal base surface. A critical distance of the channel wall from the top of the finned surface (horizontal orientation of the base surface), which causes the beginning of the heat transfer reduction, is experimentally determined, for pressures ranging from 0.5–2.0 bar.

This paper surveys methods to predict condensation and evaporation in plain, and in micro-fin and micro-channel tubes. Hydraulic diameters as small as 1.0 mm are of interest. To date, the Shah correlation has been well accepted for prediction of condensation in plain tubes. This work shows apparent deficiencies of the Shah equation for p/p_CT > 0.44. An improved predictive model called the “equivalent Reynolds number model” is introduced Use of the new model requires prediction of the single-phase heat transfer coefficient and the two-phase pressure gradient. For condensation in micro-fin tubes, both vapor shear and surface tension forces contribute to the condensing coefficient. The equivalent Reynolds number model predicts the vapor shear component. An existing theoiy of Adamek and Webb is applicable for the surface tension contribution. The vapor shear model is also applicable to evaporation inside tubes; however, it is not applicable after diyout. A nucleate boiling component will modestly add to the evaporation coefficient Data are shown that suggests the nucleate boiling contribution is less than 15%, and that this contribution exists only at vapor qualities less than 50%.

The semiconductor and its uses in transistors are briefly introduced, followed by the introduction of heat exchangers (often referred to as process chambers/reactors for IC manufacturing) for manufacturing semiconductor chips These heat exchangers can be classified, according to their functions, into six major categories: (i) heat exchangers, for the growth of thin silicon dioxide layers on a silicon wafer (also called high-temperature film furnaces), (ii) heat exchangers for doping of the masked substrate with appropriate ions (referred to as ion-implantation chambers), (iii) heat exchangers for wet/dry etching (etch chambers), (iv) heat exchangers for chemical vapor deposition (CVD reactors), (v) heat exchangers for the metallization process, and (vi) heat exchangers for heating chemical fluids. All these types of heat exchangers have one thing in common: they are all “single, solid (or tube), single-fluid” in nature, performing heat transfer by conduction, convection, or radiation. This is in sharp contrast to conventional heat exchangers in the power and refrigeration industry that consist of “double-tube (including shell-and-tube type), two-fluid” types. Therefore, heat exchangers for high-tech applications are radiative-heat-transport-controlled and are, in general easier to design and control, and environmentally clean. This lecture emphasizes how heat transfer performance can be enhanced in heat exchangers for manufacturing semiconductor chips.

The new advanced heat transfer tube called “Herringbone Heat Transfer Tube”, having a unique inner surface geometry, has been experimentally investigated with the aim to enhance the heat transfer and improve the heat exchanger performance with R407C. Experimental examinations have been carried out to obtain the heat transfer coefficient and pressure drop for R407C flowing inside the horizontal herringbone tube, and the data have been compared with those for the existing inner-grooved tube. The heat transfer enhancement mechanism for the herringbone tube has been proposed in that the thin film layer spots that occur inside the tube could cause the heat transfer enhancement. The heat exchanger performances of the herringbone tube and the standard inner-grooved tube are compared toassess thire impact on energy conservation. The heat transfer coefficients for the herringbone tube are much larger than those for the standard tube, but the significant increase in heat transfer has been accomplished with an increase in pressure drop. The circumferential local heat transfer measurements have suggested that the heat transfer mechanism proposed by the authors is qualitatively verified, but the quantitative verification of the enhancement for the herringbone tube is not sufficiently clarified. The practical comparison of the heat exchanger performance showed that the evaporator performance is improved only at relatively high refrigerant flow rates, while the condenser using the herringbone tube seems to be better than that using the standard inner-grooved tube for all the refrigerant flow rates.

A new concept air-cooled heat exchanger using mesh fins and tiny tubes, “mesh-finned heat exchanger”, has been proposed with the aim of enhancing the air-side heat transfer. The basic characteristics of the air-side heat transfer and pressure drop characteristics for the mesh finned heat exchanger have been examined experimentally, and the information required to design the practical heat exchanger has been additionally investigated, with the variation of the fin and tube arrays. The comparisons of the heat exchanger performance between the proposed heat exchangers and the existing one show that the former are superior to the latter by approximately twice. Therefore, this new concept heat exchanger can help energy conservation through heat transfer enhancement.

Because turbo engines need to increase the combustion temperature to improve their global efficiency, there have been many different devices used to protect combustion chamber walls. Multihole cooling is one of them and it has been used for a long time but the thermal mechanism needs to be studied further. A research program has been carried out and is presented here. A special test bench has been built and the temperature measurement is made using an Infra-Red camera. The main results are detailed in terms of adiabatic effectiveness and in heat transfer coefficient. The dynamic effects are also analysed to help us to understand what happens in such a difficult heat transfer situation.

Heat Exchangers are widely used at different applications related with heat science and techniques. Today at the investigation of heat exchanger projection, First Low of Thermodynamics is not anymore sufficient. The reason of this more effective use of heating sources and systems, not the quantity of energy but quality of energy must be taken into consideration. Energy transformation in the heat exchangers, in the scope of Second Law of Thermodynamics can be listed in two groups depending on chosen variables for analysis. These are a) entropy based work b) exergy based work. In this study, it is aimed to analysed theoretically and investigate experimentally effect of inlet and outlet temperatures of temperatures differences and flow directions for same path parallel flow and counter path parallel flow by using double tube heat exchanger. According to the obtained results, it is aimed to detennine necessary information for projection and to bring down to the most economical value of the heat exchanger cost and of energy consumption.

Compared to conventional heat exchangers, direct contact heat exchangers have many advantages especially in an annular geometry; because, the unequal shear stresses exerted on the liquid drops located in the annulus by the inner and outer pipe surfaces cause the deformation of drops and the break-up of drops into droplets, thus the enhancement of heat transfer is favored. This paper is a study of direct contact heat exchange between two immiscible liquids (water and oil), without phase change, in co-current turbulent flow through a horizontal concentric annulus. A correlation exists among the Nusselt number, dispersed phase volume fraction, continuous phase Weber number, annular gap and the ratio of inner radius to outer radius, considering the effects of the physical properties of the phases, the operating conditions and the geometry of the system on the overall volumetric heat transfer coefficient. A discussion of the experimental parameters and a parametric study with a calculation scheme for estimating drop size, shear stress between a drop and the continuous phase, volumetric heat transfer coefficient and the difference between wall shear stresses exerted on a drop are presented in the paper.

This paper is an edited summary of the short presentations made by the contributing Lecturers and Participants during the “Panel Discussions” session of the NATO Advanced Study Institute on “Energy Conservation through Heat Transfer Enhancement of Heat Exchangers”. The “Panel Discussions” session was organised during the Advanced Study Institute to identify unresolved problems and suggestions for future research.