Janusz T. Cieśliński

Nucleate pool boiling on porous metallic coatings

The results of systematic experimental investigations of the heat transfer to distilled water in saturated pool boiling at atmospheric pressure are reported. The tests were conducted using stainless steel tubes of various diameters heated electrically and flat horizontal plates. Surface finishes ranged from smooth lapped finish through emered and sandblasted surfaces to surfaces with porous coatings. Various methods of deposition: electrolytic treatment, plasma spraying, gas-flame spraying and modified gas-flame spraying were employed to form metal coatings on flat horizontal surfaces and on the external surface of stainless steel tubes. The main goal of this investigation was to obtain information about the parameters of the porous coatings which influence the heat transfer rate. The nucleate boiling burnout heat fluxes measured were essentially independent of surface finishes under study. Basing on the obtained experimental data published until now, mathematical models of nucleate pool boiling on porous surfaces are verified with emphasis on the prediction of the heat transfer coefficient.

Flow and pool boiling on porous coated surfaces

Abstract Industrial demands for more efficient boilers and evaporators as well as economic incentives have encouraged the development of methods to increase boiling heat transfer coefficients, critical heat fluxes, and, where possible, to obtain the highest heat flux by applying the smallest wall superheat. The goal may be to reduce the heat exchanger size or pumping power required for a specified heat duty, and also to prevent excessive temperature, or even system destruction, in systems where heat generation rates are fixed – for instance, in nuclear fuel assemblies or in chemical reactors. Numerous enhanced surfaces have been developed to intensify flow as well as pool boiling heat transfer. Excellent reviews on that topic have been provided in textbooks. Porous coated surfaces, which this article deals with, belong to the efficient categories of enhanced boiling surfaces. This article reviews the selected results of a comprehensive study of flow and pool nucleate boiling on porous coated surfaces completed in our laboratory. Particularly, this article deals with the following: (1) nucleate pool boiling on flat circular plates and horizontal tubes covered with porous coatings. Particularly, the influence of coating thickness and porosity on heat transfer coefficient and burnout heat flux for distilled water at atmospheric pressure was studied; (2) nucleate pool boiling on horizontal tube partially coated with a porous metallic layer. The possibility of porous coating application to local heat transfer enhancement, for instance in order to smooth and alleviate circumferential temperature distribution during nucleate pool boiling on horizontal, electrically heated tube, was examined; (3) nucleate pool boiling from small horizontal tube bundles built of porous coated tubes. The effect of the tube pitch and operating pressure on local, i.e., for a single tube or selected row of tubes, and average, i.e., for the whole tube bundle, heat transfer coefficient for three working fluids (distilled water, methanol, and refrigerant R141b) was studied. The bundle factor and bundle effect were determined. Second, practical application of porous coated tube bundle as an evaporator in prototype two-phase thermosyphon heat exchanger was investigated; (4) flow boiling of pure refrigerants and refrigerant/oil mixture inside porous coated tubes. The average evaporation heat transfer coefficient and simultaneous pressure drop during evaporation of R22, R134a, and R407C and their oil mixture of different concentrations was studied. A correlation equation for heat transfer coefficient calculation during flow boiling of pure refrigerants inside a tube with porous coating is proposed.

Pool Boiling of Water–Al2O3 and Water–Cu Nanofluids Outside Porous Coated Tubes

This paper deals with pool boiling of water–Al2O3 and water–Cu nanofluids on porous coated, horizontal tubes. Commercially available stainless-steel tubes having 10 mm outside diameter and 0.6 mm wall thickness were used to fabricate a test heater. Aluminum porous coatings 0.15 mm thick with porosity of about 40% were produced by plasma spraying. A smooth tube served as a reference tube. The experiments were conducted under different absolute operating pressures of 200 kPa, 100 kPa, and 10 kPa. Nanoparticles were tested at concentrations of 0.01%, 0.1%, and 1% by weight. In all cases tested, enhancement heat transfer was always observed during boiling of water–Al2O3 and water–Cu nanofluids on smooth tubes compared to boiling of distilled water. Contrary to smooth tubes, addition of even a small amount of nanoparticles resulted in deterioration of heat transfer during pool boiling of water–Al2O3 and water–Cu nanofluids on porous coated tubes in comparison with boiling of distilled water.

Influence of cylindrical screens on free convection heat transfer from a horizontal plate

Abstract Experimental investigations of laminar free convection from horizontal, screened plates to water and glycerine are reported. The screen diameter (D) was equal to the diameter of a heated isothermal plate, while the screen height (H) was varied. The effect of screening on convective heat transfer is determined. Comparison with heat transfer from an unscreened horizontal plate is made.

Free convection from horizontal screened plates

Experimental investigations of laminar free convection from horizontal isothermal surfaces screened by cylindrical vertical walls are presented. Screen diameters (D) were equal to the diameter of a heating plate while their heights (H) were varied. Results obtained for several heating fluxes and two different liquids indicate that depending on the (H/D) ratio of cylindrical screens, the heat flux transferred from the heating plate to the liquid varies. Three subranges of screening effect have been distinguished: primary inhibition, intensification and secondary inhibition. For primary inhibition, a theoretical model has been proposed and critical ratio of (H/D) has been predicted. The results of flow visualization are also discussed.ZusammenfassungEs werden experimentelle Untersuchungen der laminaren freien Konvektion an horizontalen, isothermen Flächen mit vertikalen zylindrischen Abschirmflächen beschrieben. Die Schirmdurchmesser (D) waren gleich dem einer Heizplatte, während die Schirmhöhen (H) variiert wurden. Die Resultate, welche für mehrere Wärmeflüsse und mit zwei verschiedenen Flüssigkeiten gewonnen wurden, zeigen, daß — abhängig von VerhältnisH/D der zylindrischen Schirme — der von der Heizplatte an das Fluid übergehende Wärmestrom stark variiert. Drei Bereiche konnten bezüglich des Abschirmeffektes unterschieden werden: erste Hemmung, Verstärkung und zweite Hemmung. Für den Fall der ersten Hemmung wird ein theoretisches Modell vorgeschlagen und das kritischeH/D-Verhältnis vorausberechnet. Abschließend erfolgt die Diskussion von Ergebnissen, welche aus der sichtbaren Darstellung des Strömungsfeldes gewonnen wurde.

Laminar free-convection from spherical segments

Abstract An experimental investigation of laminar free-convection heat transfer from heated, isothermal, spherical segments to a high Prandtl number liquid is reported. The effect of an extended insulator base on heat transfer is estimated. The experimentally obtained results are compared with theoretically predicted data.

Numerical Study of Turbulent Flow and Heat Transfer of Nanofluids in Pipes

ABSTRACT In this work, Nusselt number and friction factor are calculated numerically for turbulent pipe flow (Reynolds number between 6000 and 12000) with constant heat flux boundary condition using nanofluids. The nanofluid is modeled with the single-phase approach and the simulation results are compared with correlations from experimental data. Ethylene glycol and water, 60:40 EG/W mass ratio, as base fluid and SiO2 nanoparticles are used as nanofluid with particle volume concentrations ranging from 0% to 10%. Nusselt number predictions for the nanofluid are in agreement with experimental results and a conventional single-phase correlation. The mean deviation is in the range of −5%. Friction factor values show a mean deviation of 0.5% to a conventional single-phase correlation, however, they differ considerably from the nanofluid experimental data. The results indicate that the nanofluid requires more pumping power than the base fluid for high particle concentrations and Reynolds numbers on the basis of equal heat transfer rate.

Influence of Nanoparticle Concentration on Thermal Properties of Thermal Oil-MWCNT Nanofluid

The suspensions consisted of fluid phase and solid particles have been used for over 100 years. However, these tests concerned only the particles of dimension scale of mili-or micrometer, and application of such mixtures has caused serious problems that were a consequence of sedimentation of solid particles, wall erosion, clogging of the conduits or creating of sediments. The nanotechnology development has caused a vast availability of particles with dimensions less than 100 nm. Suspensions consisting of base fluid and particles (beads, filaments, plates) with dimensions less than 100 nm were given a name of nanofluids [1].

Performance of a Plate Heat Exchanger Operated with Water-Al2O3 Nanofluid

This study is focused on experimental investigation of a selected type of brazed plate heat exchanger (PHEx). The main aim of the paper was to experimentally check the ability of nanofluids to enhance the performance of PHEx. A typical water-Al2O3 nanofluid was tested and compared to that of the base fluid, i.e. water. Nanoparticles were tested at the concentration of 0.1% and 1% by weight. Impact of the 1 day and 3 days break of operation of the tested PHEx on its performance was of particular interest. Pressure drop in all runs was measured as well. The Wilson approach was applied in order to estimate heat transfer coefficients for the PHEx passages. It was observed, that addition of nanoparticles resulted in deterioration of an overall heat transfer coefficient for the selected PHEx and tested conditions, i.e. temperature range and Reynolds number. Moreover, substantial increase of pressure drop was recorded after each break of operation of the tested PHEx.

Performance of the Flat Plate Solar Collector Operated with Water-Al2O3 Nanofluid

Solar collectors is one of the technologies absorbing energy from solar beam and utilizing it for heating purposes, displacing the need to burn fossil fuels. There are many ways to improve effectiveness of the solar collectors [1,2]. Recent method to absorb more heat from the solar beam is to modify thermal characteristics of the working fluid. For this purpose one can use nanofluids, i.e. suspensions of metallic or nonmetallic nanoparticles in a base fluid [3].