Boiling heat transfer characteristics of nanofluids jet impingement on a plate surface

The jet boiling heat transfer of a bar water–CuO particle suspensions (nanofluids) jet impingement on a large flat surface was experimentally investigated. The experimental results were compared with those from water. The quantificational effects of the nanoparticles concentration and the flow conditions on the nucleate boiling heat transfer and the critical heat flux (CHF) were investigated. The experimental data showed that the jet boiling heat transfer for the water–CuO nanofluid is significantly different from those for water. The nanofluids have poor nucleate boiling heat transfer compared with the base fluid due to that a very thin nanoparticle sorption layer was formed on the heated surface. The CHF for the nanofluid increased compared with that of water. The reasons were that the solid–liquid contact angle decreased due to a very thin sorption layer on the heated surface and the jet and agitating effect of the nanoparticles on the subfilm layer enhance supply of liquid to the surface.     

Boiling characteristics in small vertical tubes with closed bottom for nanofluids and nanoparticle-suspensions

An experimental study was carried out to understand the nucleate boiling characteristics and the critical heat flux (CHF) of water, the water based nanofluids and the water based nanoparticle-suspensions in vertical small heated tubes with a closed bottom. Here, the nanofluids consisted of the base liquid, the CuO nanoparticles and the surfactant. The nanoparticle-suspensions consisted of the base liquid and CuO nanoparticles. The surfactant was sodium dodecyl benzene sulfate. The study focused on the influence of the nanoparticles and surfactant on the nucleate boiling characteristics and the CHF. The experimental results indicated that the nanoparticle concentrations of the nanofluids and nanoparticle-suspensions in the tubes do not change during the boiling processes; the nanoparticles in the evaporated liquid are totally carried away by the steam. The boiling heat transfer rates of nanofluids are poorer than that of the base liquid. However, the boiling heat transfer rates of nanoparticle-suspensions are better than that of the base liquid. Comparing with the base liquid, the CHF of the nanofluids and the nanoparticle-suspensions is higher. The CHF is only related to nanoparticle mass concentration when the tube length and the tube diameter are fixed. The experiment confirm that there is a thin nanoparticle coating layer on the heated surface after the nanofluids boiling test but there is no coating layer on the heated surface after the nanoparticle-suspensions boiling test. This coating layer is the main reason that deteriorates the boiling heat transfer rates of nanofluids. An empirical correlation was proposed for predicting the CHF of nanofluids boiling in the vertical tubes with closed bottom.     

Circumferential temperature distribution during nucleate pool boiling outside smooth and modified horizontal tubes

In the work an approach to avoid a circumferential temperature distribution existing during nucleate pool boiling on a horizontal cylinder within low heat flux densities is presented. The idea of the approach is local heat transfer enhancement by a porous layer application on a part of the heating surface. An experiment on nucleate pool boiling heat transfer from horizontal cylinders to saturated R141b and water under atmospheric pressure is reported. Experiments have been conducted using stainless steel tubes with the outside diameter between 8 mm and 23 mm with the active length of 250 mm. The outside surface of the tubes was smooth or partially coated with a porous metallic layer. In particular, measurements of inside circumferential temperature distribution have been performed.     

Experimental investigation of circular free-surface jet impingement quenching: Transient hydrodynamics and heat transfer

An experimental study of heat transfer during quenching of a cylindrical stainless steel test specimen has been performed. A subcooled water jet is directed onto the upward facing flat face of the cylinder. The test specimen is heated to an initial temperature slightly above 900 °C and then quenched. The resulting boiling curves and heat transfer distributions are presented for impingement velocities of 2.85 and 6.4 m/s (Re = 7900 and 18,900). High-speed imaging shows that three distinct regions on the quenched surface can be identified: an expanding circular wetted region surrounding the impinging point, annular transition zone just outside the wetting front, and a unwetted region outside this zone. The free-surface of the liquid in the wetted region is smooth in the nucleate and transition boiling regimes. The annular transition zone or the wetting front region outside the wetted region is characterized by a highly disturbed liquid-gas interface, which can be attributed to intense vapor generation. At the outer edge of the transition zone, the liquid is deflected away from the surface. The velocity of the wetting front significantly increases with the jet impact velocity, which indicates that the wetting front position is governed by the ability of the flowing liquid to transport the bubbles radially outwards from the wetted region.     

Analytical considerations of flow boiling heat transfer in metal-foam filled tubes

Flow boiling in metal-foam filled tube was analytically investigated based on a modified microstructure model, an original boiling heat transfer model and fin analysis for metal foams. Microstructure model of metal foams was established, by which fiber diameter and surface area density were precisely predicted. The heat transfer model for flow boiling in metal foams was based on annular pattern, in which two phase fluid was composed by vapor region in the center of the tube and liquid region near the wall. However, it was assumed that nucleate boiling performed only in the liquid region. Fin analysis and heat transfer network for metal foams were integrated to obtain the convective heat transfer coefficient at interface. The analytical solution was verified by its good agreement with experimental data. The parametric study on heat transfer coefficient and boiling mechanism was also carried out.     

Dynamic Evolution of an Evaporating Liquid Meniscus from Structured Screen Meshes

Evaporation from screen mesh is a fundamental phenomenon that plays a vital role in thermal transport devices like heat pipes. Here, a laser scanning confocal microscopy setup has been utilized to study the effect of screen mesh wettability and its surface morphology on the evolution of evaporating liquid menisci from its liquid saturated pores. Stainless steel screen meshes (mesh #100 and #200) that are inherently hydrophobic in nature are turned super-hydrophilic by controlled heat treatment. The heat treatment leads to growth of oxide layer on the wire mesh and a change in surface morphology via formation of microscale pores, which improves the wettability of the screen mesh. The evaporation of liquid meniscus from pores of these untreated and heat-treated meshes is captured through confocal microscopy, and the dynamic evolution of the radius of curvature of the liquid meniscus is evaluated. A simple geometrical model is developed to predict the minimum radius of curvature \((R_{\mathrm{min}})\) of liquid meniscus in mesh pores just before its rupture (pore dryout). Meshes with high wettability, and smaller pore spacing-to-wire diameter ratio, are found to encounter a smaller \(R_{\mathrm{min}}\) before meniscus rupture. In addition, the water pore saturation inventory of the screen meshes are also measured to evaluate the effect of wettability on their water-holding capacity of screen meshes. Increase in screen mesh wettability is found to increase its pore saturation inventory. This increase in mesh pore saturation inventory coupled with the lower \(R_{\mathrm{min}}\) for super-hydrophilic mesh delays the liquid meniscus rupture (mesh dryout), leading to a much longer evaporation timescale for high wettability screen meshes.     

Temperature fluctuations in a two-phase superheated layer in the boiling of a liquid

One of the basic parameters defining the intensity of heat transfer on the boundary separating liquid and vapor phases is the superheating of the liquid during boiling. In a two-phase boundary layer superheating constitutes a variable (fluctuating) quantity, which depends on several parameters of the system and is of a statistical nature. This paper is devoted to a study of the statistical nature of the fluctuations in the superheating of a liquid. Starting from experimental data, obtained in measuring temperature fluctuations in a two-phase boundary layer during the boiling of water in contact with a heated surface, we carry out a statistical analysis of the amplitude of the fluctuations. Based on this analysis, we determine the average and the maximum superheating as a function of the distance to the heated wall. To determine the microstructure of the temperature fluctuations and to study their origin, we took high-speed pictures of the head of a thermocouple in contact with the two-phase medium. We established that the presence of various size amplitudes is associated, in the main, with two effects: the existence of a superheated layer on a bubble in the course of its growth and the convection of the liquid close to the bubble.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 62–69, September–October, 1973.     

Crystallization fouling of finned tubes during pool boiling: effect of fin density

Bubble characteristics such as density, size, frequency and motion are key factors that contribute to the superiority of nucleate pool boiling over other modes of heat transfer. Nevertheless, if heat transfer occurs in an environment prone to fouling, the very same parameters may lead to accelerated deposit formation due to concentration effects beneath the growing bubbles. This has led to the widely accepted design recommendation to maintain the heat transfer surface temperature below the boiling point if fouling may occur, e.g., in seawater desalination. The present paper aims at investigating the formation of deposits on finned tubes during nucleate pool boiling of CaSO₄ solutions. The test finned tubes are low finned tubes with fin densities of 19 and 26 fins/in. made from Cu–Ni. The fouling experiments were carried out at atmospheric pressure for different heat fluxes ranging from 100 to 300 kW/m² and a CaSO₄ concentration of 1.6 g/L. For the sake of comparison, similar runs were performed with smooth stainless steel tubes. The results show that: (1) the fouling resistance decreases with increasing fin density, (2) fouling on the finned tubes was reduced with increasing nucleate boiling activity and (3) if any fouling layer occurred on the finned tubes it could be removed easily.     

The bubble fossil record: insight into boiling nucleation using nanofluid pool-boiling

Subcooled pool boiling of Al₂O₃/water nanofluid (0.1 vol%) was investigated. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to observe surface features of the wire heater where nanoparticles had deposited. A layer of aggregated alumina particles collected on the heated surface, where evidence of fluid shear associated with bubble nucleation and departure was “fossilized” in the fluidized nano-porous surface coating. These structures contain evidence of the fluid forces present in the microlayer prior to departure and provide a unique understanding of boiling phenomena. A unique mode of heat transfer was identified in nanofluid pool boiling.     

Heat transfer from horizontal tubes in pool boiling: influence of three-dimensional heat conduction in the wall of the evaporator tube—a finite element analysis

In pool boiling, the electrically heated tube releases the energy non-uniformly to the liquid, due to different surface roughness and flowing liquid. The heat transfer coefficient therefore varies with axial and azimuthal position on the tube. Hence a finite element analysis has been carried out on a horizontal 1in. copper tube for evaporation in pool boiling for three-dimensional conduction heat transfer. A test tube has been made with different surface structures, tested and analysed for heat conduction effects. It has been observed that significant amount of heat flows in azimuthal and axial directions in addition to the heat flow in radial direction.     

Preparation and analysis of different roughness structures for evaporator tubes

The processes of the phase change in boiling occur at the solid–liquid interface by heat transfer from a solid heating surface to the boiling liquid. The characteristic features of the heating surfaces are therefore of great interest to optimize the design of evaporators. The microstructure with all its peaks and cavities influences directly the wetting and rewetting conditions of the heated surface by the boiling liquid and hence bubble formation and heat transfer. The roughness structures of different evaporator copper tubes with 8 or 25 mm diameter are characterized quantitatively with regard to the cavities offered to nucleation. The surfaces of the heating elements are sandblasted by different means resulting in a stochastic microstructure. The surfaces are investigated by a three-dimensional contactless roughness measurement technique combining the stylus technique with the near field acoustic microscopy. The method opens the possibility to obtain results according to standard for practical applications and additionally delivers detailed information about the three-dimensional shape of each cavity within the surface investigated. The analysis of the microstructure implies the total number of cavities, their local and size distribution calculated by the method of the envelope area. The results of the surface analysis are linked to those of heat transfer and bubble formation discussed in a contribution by Kotthoff and Gorenflo.     

Heterogeneous nucleation on ultra smooth surfaces

An experimental investigation is presented with heterogeneous nucleate boiling on ultra smooth metallic surfaces (30-365 nm RMS roughness), including brass, unpolished stainless steel, and electropolished stainless steel. The fluids used for the investigation are highly wetting pentane and butane. It is observed that the incipient superheat is low for all cases considered, despite the fact that no vapor trapping cavities are available for incipience at low superheat. These data provide further evidence that in addition to vapor trapping, another mechanism must be available for heterogeneous nucleation in boiling systems. The boiling curves are presented for the different surface/fluid combinations. It is found that the heat transfer rate on the brass surface is considerably better than that on the stainless steel surfaces due to the ease with which nucleation sites are formed. In contrast, nucleation site formation on stainless steel is considerably more sparse than that on brass.     

Explosive boiling of water in parallel micro-channels

The objective of this study is to visualize the flow pattern and to measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels. Tests were performed in the range of inlet Reynolds number 25–60, mass flux 95–340 kg/m²s, and heat flux 80–330 kW/m².The flow visualization showed that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains. This process may be regarded as explosive boiling with periodic wetting and dryout.In the presence of two-phase liquid–vapor flow in the micro-channel, there are pressure drop oscillations, which increase with increasing vapor quality.This study shows strong dependence of the heat transfer coefficient on the vapor quality. The time when liquid wets the heated surface decreases with increasing heat flux. Dryout occurs immediately after venting of the elongated bubble.     

Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface

Enhancements of nucleate boiling critical heat flux (CHF) using nanofluids in a pool boiling are well-known. Considering importance of flow boiling heat transfer in various practical applications, an experimental study on CHF enhancements of nanofluids under convective flow conditions was performed. A rectangular flow channel with 10-mm width and 5-mm height was used. A 10 mm-diameter disk-type copper surface, heated by conduction heat transfer, was placed at the bottom surface of the flow channel as a test heater. Aqueous nanofluids with alumina nanoparticles at the concentration of 0.01% by volume were investigated. The experimental results showed that the nanofluid flow boiling CHF was distinctly enhanced under the forced convective flow conditions compared to that in pure water. Subsequent to the boiling experiments, the heater surfaces were examined with scanning electron microscope and by measuring contact angle. The surface characterization results suggested that the flow boiling CHF enhancement in nanofluids is mostly caused by the nanoparticles deposition of the heater surface during vigorous boiling of nanofluids and the subsequent wettability enhancements.     

Flow boiling behaviors in hydrophilic and hydrophobic microchannels

Surface wettability is a critical parameter in small scale phenomena, especially two-phase flow, since the surface force becomes dominant as size decreases. In present study, experiments of water flow boiling in hydrophilic and hydrophobic rectangular microchannels were conducted to investigate the wettability effect on flow boiling in rectangular microchannels. The rectangular microchannels were fabricated with a photosensitive glass to visualize flow pattern. The hydrophilic bare photosensitive glass microchannel was chemically treated to obtain a hydrophobic microchannel. And, visualization of flow patterns was carried out. And boiling heat transfer and two-phase pressure drop was analyzed with visualization results. The boiling heat transfer coefficient in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with nucleation site density and liquid film motion. And the pressure drop in the hydrophobic rectangular microchannel was higher than that in the hydrophilic rectangular microchannel, which was highly related with unstable motions of bubble and liquid film. Finally, we find out the wettability is important parameter on the flow pattern, which were highly related with two-phase heat and mass transfer.     

Two-phase structure above hot surfaces in jet impingement boiling

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found.     

Einfluß der Heizflächenbeschichtung auf den Wärmeübergangskoeffizienten beim Behältersieden von Wasser,Isopropanol, Azeton und deren Mischungen

Zusammenfassung In der vorliegenden Arbeit wird der Wärmeübergang von drei Stahlheizstäben mit unterschiedlicher Oberfläche verglichen. Ein Heizstab wurde mit einer perforierten Messingfolie umhüllt, der zweite mit Teflon beschichtet, während der dritte unbeschichtet verwendet wurde. Als Testflüssigkeiten wurden Wasser, Azeton, Isopropanol und deren Mischungen verwendet.

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The influence of a coating of the heating surface on the heat transfer coefficient during pool boiling of water, isopropanol, acetone and their mixtures

This article compares pool boiling heat transfer from three stainless steel heating rods with different surfaces. One heating rod was covered with a perforated brass foil, one was coated with teflon and the third was used without any coating. Water, acetone, isopropanol and their mixtures were used as test liquids.

     

Studies on heat transfer and flow characteristics in subcooled flow boiling—Part 2. Flow characteristics

Distributions of fluid temperature and its fluctuation are measured across a R-113 subcooled boiling flow channel with heat fluxes up to the CHF. A microthermocouple probe associated with an electric compensation circuit for the time constant is used for this purpose. Applying statistical treatments to the recorded temperature fluctuation, the heat transfer process in the flow and the characteristics of the bubbles flowing close to the heated surface are investigated. For high heat fluxes nearby the CHF, some bubbles adjacent to the heated surface show a clear trend to coalesce to large volume bubbles with relatively long passing periods, suggesting a mechanism of departure from nucleate boiling by periodical wall temperature rise due to momentary liquid film dryout underneath the large bubbles.     

Studies on heat transfer and flow characteristics in subcooled flow boiling—Part 1. Boiling characteristics

Measurement of wall temperature profile and photographic observation are performed for R-113 subcooled boiling flow in a channel with heat fluxes up to the CHF. The incipient boiling superheats measured are little affected by mass velocity and liquid subcooling. Hysteresis in boiling observed by increasing and decreasing heat flux seems to be ascribed to variation in size of active nucleation cavities on the wall. Increasing heat flux up to the CHF, the bubble density on the heated surface increases and remarkably large coalescent bubbles appear periodically near the heating section outlet.     

Heat transfer in pool boiling of binary and ternary non-azeotropic mixtures

Heat transfer coefficients in nucleate pool boiling of binary and ternary non-azeotropic hydrocarbon mixtures were obtained experimentally using a vertical electrically heated cylindrical carbon steel surface at atmospheric pressure with several surface roughness. The fluids used were Methanol/1-Pentanol and Methanol/1-Pentanol/1,2-Propandiol at constant 1,2-Propandiol mole fraction of 30%. Heat fluxes were varied in the range 25–235 kW/m². The cylindrical heater surface was polished to an average surface roughness of 0.2 μm, and sandblasted yielding surface roughness of 2.98 and 4.35 μm, respectively. The experimental results were compared to available prediction correlations, indicating that the correlations based on the boiling range are in better qualitative agreement than correlations based on the phase envelope. Increasing surface roughness resulted in an increase in the heat transfer coefficient, and the effect was observed to be dependent on the heat flux and fluid composition.