Heat Transfer Surface Temperature Research Articles

Effect of ZnO-water based nanofluids from sonochemical synthesis method on heat transfer in a circular flow passage

Present research work highlighted the synthesis of ZnO Nanoparticles by sonochemical method and its positive effects on enhancement of heat transfer while used as water based nanofluids in a circular tube heat exchanger. ZnO nanoparticles were synthesized by using Zinc acetate (ZnC4H6O4) and Sodium Hydroxide (NaOH) as the raw materials. Proper formation of ZnO nanoparticles were confirmed by XRD, FTIR, FESEM, EDX mapping and UV–vis spectrum characterizations. ZnO nanoparticles were been dispersed in distilled water by uisng high probe sonication and its concentration was varied (0.1%, 0.075%, 0.05% and 0.025%) to study its effect on convective heat transfer (Nusselt number) with the variation of Reynolds number in single tube circular heat exchanger. Positive enhancement in thermal conductivity was observed with the addition of ZnO nanoparticles in the base fluid and studied its performance at 50 °C heat transfer surface temperature and 30 °C bulk temperature of the nanofluids. About 49% increase in Nusselt numbers was observed at 0.1% wt. concentration of ZnO-water based flowing Nano fluid. At the highest Reynolds number (Examined), there was about 50% heat transfer enhancement noticed at 0.1% concentration of ZnO nanofluid. While all other weight % concentrations also showed enhanced heat trasnfer properties as compare to base fluid. The ZnO with distilled water nanofluids gives encourging results for heat trnasfer improvments.

The single-blow transient test technique using pulse change inlet condition with optimized pulse width and matching time

Compact heat exchanger is a kind of advanced heat transfer equipment with small size and high efficiency. It has wide application prospects in industry. However, due to the highly compact structure, it is difficult to measure the wall temperature of heat transfer surface by traditional test methods. For measuring the thermal performance of compact heat transfer surfaces more accurately, a single-blow transient test technique using pulse change inlet condition with optimized pulse width and matching time is developed. By turning on and off the electric air heater, the pulse change in the inlet temperature can be realized and is fitted as the superposition of a positive and a negative exponential function with a time shift. In order to reduce the effect of the uncertainty of temperature measurement, the optimum pulse width and optimum matching time is obtained by numerical calculations. By means of the newly extended test method, the heat transfer performance of a parallel-plate test core is measured and compared with the results from the literature. The analysis shows that the present pulse change technique considering the optimal pulse width and matching time have to be considered for the heat transfer surfaces with NTU > 4.5 to reduce the uncertainty in temperature measurement. The experimental results are in good agreement with those given in the literature.

Experimental study on the sticking probability and deposit bond strength of fouling in enhanced tubes

Sticking probability (P) and deposit bond strength (ξ) are two of the most important factors that determine the fouling process, but no calculation correlations for both P and ξ could be found in current models. Thus, the fouling process could not be described by specific formulas. This paper analyzed each parameter in the Kern-Seaton fouling model, and a correlation of dry matter concentration (Cb′) was developed, depending on the water quality test. Test data suggested that the sticking probability (P) in such combined fouling ranged from 0.96445×10−3 to 6.20781×10−3. The deposit bond strength (ξ) of fouling ranged from 0.99953×107 N·s/m2 to 3.51186×107 N·s/m2. Furthermore, the calculation correlations of P and ξ in types of f(Re,Ns,α,e/Di) and f(f,j) for two-dimensional enhanced tubes were developed, respectively, based on long-term fouling data. Results indicated that the sticking probability (P) was consistently decreasing in both the j-factor and frictional factor (f), but it was consistently increasing in the temperature of heat transfer surface (T). Both the sticking probability (P) and deposit bond strength (ξ) were more affected by frictional drag performance of the tubes than the heat transfer performance. The sticking probability (P) was negatively related to the specific geometric parameters (Ns, α, e/Di) of the tubes, but positively related to the Reynolds number, Re, which was completely opposite to the deposit bond strength (ξ). The parameters of e/Di and α affected both the sticking probability (P) and the deposit bond strength (ξ) more than the starting number of Ns.

Deposit Build-up and Corrosion in a Copper Flash Smelting Heat Recovery Boiler

The aim of the work was to improve the understanding of deposit formation and corrosion in a copper flash smelting plant, focusing on the effects of process gas temperature (400–900 °C) and heat-transfer surface temperature (160–320 °C) on deposit formation and corrosion rate. The rate of build-up increased as a function of process gas temperature, which was explained by a larger extent of sintering at higher temperatures, resulting in slagging and thus, in better adhesion of particles hitting the surface. The corrosion rate increased as a function of process gas temperature. Iron sulphate (FeSO4 or Fe2(SO4)3) was found at the interface between the deposit and the corroded metal surface, suggesting that corrosion was induced by sulphuric acid (H2SO4) from the reaction between SO3 (originating from SO2 in the process gas) and water vapour.

Study of cycle output improvement by work-fluid including phase change material

This paper deals with the output improvement of heating and cooling cycle by using the work-fluid including phase change material. The experimental study is carried out by heat exchange between work-fluid and heat transfer surface. The work-fluid is flown to a high temperature or a low temperature heat transfer surface from the narrow path. In order to increase the amount of the heat transmission, a trace of Diethylether (boiling point 34.8 °C), as a phase change material (PCM), is added to the work-fluid. The parameters of the experiment are additive amount of PCM, the rotational speed of the displacer piston and the temperature of heat transfer surface. It is clarified that the increasing of engine cycle output is brought by the PCM addition. The effect of PCM addition is evaluated by output ratio which is defined from the experimental cycle output data. The requirements for acquiring the increasing effect of output by adding PCM are clarified.

Nanofluids for power engineering: Emergency cooling of overheated heat transfer surfaces

The possibility of emergency cooling of an overheated heat transfer surface using nanofluids in the case of a boiling crisis is explored by means of synchronous recording of changes of main heat transfer parameters of boiling water over time. Two nanofluids are tested, which are derived from a mixture of natural aluminosilicates (AlSi-7) and titanium dioxide (NF-8). It is found that the introduction of a small portions of nanofluid into a boiling coolant (distilled water) in a state of film boiling (t heater > 500°C) can dramatically decrease the heat transfer surface temperature to 130–150°C, which corresponds to a transition to a safe nucleate boiling regime without affecting the specific heat flux. The fact that this regime is kept for a long time at a specific heat load exceeding the critical heat flux for water and t heater = 125–130°C is particularly important. This makes it possible to prevent a potential accident emergency (heater burnout and failure of the heat exchanger) and to ensure the smooth operation of the equipment.

Heat Transfer Characteristics of Work Fluid Including Phase Change Material That Flow into Heating Surface from Narrow Path

Use of the low temperature (less than 100°C) energy contributes to effective use of heat resources. The cost recovery by power generation is difficult by using an existing system (the binary cycle or the thermoelectric conversion element), because the initial investment is large. The final purpose of this research is development of the low temperature difference drive engine supposing use in a hot-springs resort as a power source for electric power generation. In order that a traveler may look at and delight a motion of an engine, it is made to drive at low-speed number of rotations. An engine cycle of this study is aimed at the development of Stirling cycle engine which can maintain high efficiency in small size. This kind of engine has simple structure; it brings low cost, and it is easy to perform maintenance. However, it is difficult to obtain enough output by this type of engine, because of its low temperature difference. This paper deals with the heat transfer characteristic that the working fluid including a phase change material flows into the heating surface from the narrow path. In order to increase the amount of the heat transmission, Diethylether is added to the working fluid. Diethylether is selected as a phase change material (PCM) that has the boiling point which exists between the heat source of high temperature and low temperature. The parameters of the experiment are additive amount of PCM, rotational speed of the displacer piston and temperature of heat transfer surface. It is shown that it is possible to make exchange of heat amount increase by adding phase change material. The result of this research shows the optimal condition of the difference in temperature in heat processing, number of revolutions, and addition concentration of PCM.

Prediction of micro surface cooler performance for different rectangular type microchannels dimensions

In this publication the investigation of the heat transfer characteristics and pressure drop in a micro surface cooler for different microchannel geometries by CFD simulation is described. The simulation is considering only a single row of microchannels with laminar flow, taking advantage of symmetry effects for the total micro surface cooler device. A comparison between simulation and experimental results has been carried out. The maximum temperature deviation of 10K has been found from the comparison study, especially for higher heat flux applied. Changing the microchannel depth and width results in a significant effect to cooling capability and the pressure drop of the system. A microchannel depth decreased by a factor of two provides a heat transfer coefficient increase of about 43% for devices made of polymer. At the same time, the heat transfer surface temperature is decreased by ten degrees.This study is expected to lead to a guideline for the optimization of micro surface coolers with short microchannels.

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.

Experimental Study of Fouling on Heat Transfer Surface During Forced Convective Heat Transfer

An experimental study was conducted to investigate the fouling process of calcium carbonate on the heat transfer surface, during forced convective heat transfer. The dynamic monitoring apparatus of fouling resistance was set up for the present experiments. The fouling behaviors were examined under different factors including fluid velocity, hardness, alkalinity, solution temperature, and wall temperature. Asymptotic fouling curves varying with time were obtained. The fouling rate and asymptotic fouling resistance increased and the induction periods were shortened with the fluid velocity decreasing, hardness and alkalinity increasing, and solution temperature and heat transfer surface temperature increasing. The components of fouling that formed on the heat transfer surface included crystallization fouling and particulate fouling. The thermal performance parameter of fouling, ρ fλ f, varied from 380 to 2600 kg·W·(m 4·K) −1, increasing with growing velocity and decreasing solution temperature, hardness or alkalinity. Furthermore, the thermal conductivity of fouling, λ f, varied from 1.7 to 2.2 W·(m·K) −1.

Effects of process parameters and anti-scalants on scale formation in horizontal tube falling film evaporators

In multiple-effect distillers with horizontal tubes, which are increasingly employed in seawater desalination, scale formation on the heat transfer surfaces has a highly deleterious effect on specific energy consumption and production capacity. The allowance for potential scale formation by over-sizing the heat transfer surface, scale prevention methods, cleaning methods as well as production losses during plant shutdown for cleaning create considerable capital, operating, and maintenance costs. It is therefore essential to incorporate a cost-effective scale control technique. In multiple-effect distillers scale is formed on the outside of the tubes. Mechanical cleaning methods cannot be employed. Achieving effective scale control by suitable anti-scalants is a major challenge for the desalination industry. Scale formation and control in multiple-effect distillers with horizontal tubes for seawater desalination have barely been investigated. The effects of the process parameters and of the anti-scalants on the properties of the scale, e.g., the composition, structure, and morphology, are inadequately understood. Clarifying these aspects of scale formation would facilitate further improvement of anti-scalants and thus the efficiency of multiple-effect distillers. In order to study scale formation and control under conditions that are close to those prevailing in industrial multiple-effect distillers, a horizontal tube evaporator at pilot-plant scale was employed. In the test rig the process conditions in any stage and at various operating conditions of multiple-effect distillers can be provided. Thus, the effects of the thermal field, the flow field, the fluid properties, and the substrate properties can be studied. Experiments with artificial seawater were performed at different operating conditions. Novel aspects of the formation and reduction of scales formed from falling seawater films on the outside of horizontal tubes are presented. The effects of various process conditions such as fluid temperature, heat transfer surface temperature, heat flux, and seawater salinity on the composition and structure of scale layers are shown. The beneficial effects of polymeric anti-scalants on the quantity, composition, and morphology of the scale are discussed. For example, the use of only 0.5 ppm of a specific anti-scalant effectuated a very strong reduction of the calcium carbonate content in the scale as well as a notable decline of the magnesium hydroxide content.

Which are the right test conditions for the simulation of high temperature alkali corrosion in biomass combustion?

AbstractThe use of renewable energy sources for the generation of electricity has gained much interest in recent years. Biomass has the potential of being a CO2 neutral energy source that could provide a significant proportion of the ever‐increasing energy demand. The easiest and most straightforward utilization of the energy content of biomass is direct combustion. The thermal energy of the biomass released in the combustion process is utilized by heating water or steam to produce electricity or heat or both (CHP). Whereas the heat production requires only modest temperatures in the water or steam circuit, producing electricity with high efficiency is not possible without high steam parameters (temperature and pressure). The heat market being quite saturated, the economic potential is in the electricity market. High heat transfer surface temperatures coupled with biomass fired steam generators have resulted, however, in serious corrosion of the heat transfer surfaces, especially in the hottest section of the convection superheaters. The type of corrosion found in biomass boilers is not encountered in fossil fuel fired boilers and the mechanism causing it is not fully understood. The development of new alloys that could withstand these harsh environments would benefit tremendously if the test conditions in the laboratory tests could be chosen so that they adequately resemble the corrosion environment in real boilers. Currently the high corrosion rate is believed to be caused by gaseous KCl that condense on the heat transfer surfaces. While KCl is certainly found in the corroded superheater tubes and probably has an important role in the corrosion reactions with the alloy, the formation of KCl on the cooled surface can also be heterogeneous. In this paper a discussion on the effect of alkali hydroxides, especially KOH, is presented. Biomass fuels have normally a high content of alkali metals and a low content of sulfur and chloride. The excess alkali will produce alkali hydroxides in the combustion environment. Alkali hydroxides then react with CO2 in the flue gases to form carbonates as the flue gases are cooled. The reaction with CO2, is however, very temperature dependent. The equilibrium being completely on the K2CO3 side with a gas temperature below 700 °C and completely on KOH side with a gas temperature above 900 °C. The hottest superheaters are normally located in the area where the flue gas temperature is 850°C–1000 °C. This makes KOH condensation on the tubes possible and subsequent heterogeneous reactions with HCl, SO2 and CO2 in molten phase forming KCl, K2SO2 or K2CO3. Although KOH is not thermodynamically stable at typical tube surface temperatures, a continuous flux condensing from the flue gases results in a corrosion environment on the tube where its activity has to be taken into account. Therefore it is suggested that KOH, either in gaseous or molten phase should be included in the laboratory test environments used for the testing of alloys for biomass combustion applications.

1202 減圧下におけるスプレー冷却熱伝達特性について(第2報)(GS-6 強制対流・沸騰,研究発表講演)

Many researchers have studied about spray coolings and obtained the results of measurement. In this research, the purpose is to improve the performance of cooling in low temperature regime, and to obtain the fundamental database for the highly precision control of heat transfer surface temperature. The experiments on steady cooling on downward horizontal ribbon were performed. The heat transfer process and cooling limitation heat flux of spray were investigated. The experimental results for two kind of spray nozzles were obtained for flow rates and subcoolings. The correlation of cooling limitation heat flux was presented.

619 減圧下におけるスプレー冷却熱伝達特性について(GS-6 熱工学(2))

Many researchers have studied about spray coolings and obtained the results of measurement. In this research, the purpose is to improve the performance of cooling in low temperature regime, and to obtain the fundamental database for the highly precision control of heat transfer surface temperature. The experiments on steady cooling on horizontal cylinder with upward water spray were performed. The heat transfer processes and heat flux limitation of spray cooling were investigated. The results of experiment for various spray nozzles were shown for flow rates and subcoolings..

Heat Transfer from a Finned Surface to Boiling Liquid Mixtures

A numerical study of heat transfer to acetone / isopropanol mixtures boiling on a finned surface was performed using a finite-difference technique. Heat transfer surface temperature, liquid composition, fin geometry, and fin thermal conductivity have been varied to investigate the influence of these parameters on fin-average heat transfer coefficients and on the amount of heat transferred by the finned surface. The results indicate that the significant degradation of heat transfer that is found for boiling of mixtures on plain surfaces can be reduced by using finned heat transfer surfaces.

Understanding and mitigating heat exchanger fouling in bauxite refineries

Laboratory and plant measurements have been performed to evaluate possibilities of reducing the formation of desilication product (DSP) scale in the heat exchangers of the low-temperature Bayer process. It was found that the rate of scale formation is independent of the flow velocity, proportional to the square of the silica super saturation, and it increases exponentially with heat-transfer surface temperature. Because of the restricted operation conditions, variation of operating conditions may not reduce DSP formation sufficiently. Installation of turbulence promoters in the heat exchanger pipes had no beneficial aspects. Laboratory and plant measurements with a fluidized bed test heater indicated a strong potential of this technology to increase the operation times between chemical washes. Deposition of DSP could almost be eliminated if the process was modified such that developed subcooled boiling occurred at the heat-transfer surface.

スートファイア (排ガスエコノマイザメルトダウン事故) の発生機構

The mechanism of soot-fire accident of exhaust gas economizer in diesel ship is investigated in this report. This is the second report following the first one, in which the process of soot-burning of exhaust gas economizer was studied by a simulation model. According to the soot-burning simulation, temperature of heat transfer surface seldom reaches its melting point only by soot-burning. So it is difficult to consider that soot-fire is occurred only by heat released by the soot-burning. It has been reported the oxidative heat of tube materials might be another heat source. The experiments, therefore, are carried out in order to measure the rate of oxidation of tube materials, and reveal that there is a possibility that the oxidative heat might be another heat source when the oxidation rate is accelerated by the high temperature steam atmosphere and the“break-away”oxidation. Accordingly it is made clear the soot-fire is occurred by a series of events, that is, burning of soots deposited on finned tubes, waterside dry-out of the finned tubes induced by the soot-burning, sudden degradation of waterside heat removal due to the dry-out, rapid increase of fin temperature, and further increase of the fin temperature due to oxidation of iron, and finally melt-down of the heat transfer surfaces.

The dependence on heat-transfer-surface-temperature of the enhancement of heat transfer by corona wind

It is shown experimentally that the enhancement of heat transfer from a heat transfer surface by corona wind drops as the temperature of the heat transfer surface increases due to an increase in the power supplied to the heat transfer surface. The cooling effect of the corona wind is likewise affected. Measurements are reported where an increase of a heat transfer surface temperature by about 100°C reduces the enhancement of heat transfer by corona wind from about 300 percent to only 50 percent. This behaviour is explained by the more than linear rise of the coefficient of natural heat convection with respect to the temperature rise above ambient of the heat transfer surface and also due to an increase in heat transfer by radiation from the heat transfer surface.

Post-dryout heat transfer of high-pressure steam-water two-phase flow in single rod channel and multi rod bundle

Low flow rate post-dryout heat transfer experiments of steam-water two-phase flow were conducted at the Japan Atomic Energy Research Institute using single rod and 25 rod heat transfer test facilities. The experiments covered mass flow rates from 20 to 800 kg/m 2s, qualities from 0.0 to 1.0 and pressures from 3 to 12 MPa. New correlations for the post-dryout heat transfer coefficient have been proposed by improving the Groeneveld correlation. The proposed correlations can account for the dependence of the heat transfer on the heat transfer surface temperature which the original Groeneveld correlation cannot do explicitly. The agreement between the predicted and measured values is satisfactory.

Heat Transfer to Separated Flow Regions From a Rectangular Prism in a Cross Stream

A series of experiments have been performed to determine the constant surface temperature heat transfer and the corresponding surface pressure variation on a two-dimensional rectangular prism in crossflow whose aspect ratio (chord to height) was 4/1. Results are presented for a surface exposed to both separated and reattached flow and for a wake exposed surface, each for several different angles of attack. Universal Stanton number and pressure coefficient curves result when data are plotted against downstream distance scaled by the length of the near-constant pressure portion of the separated bubble for the reattaching case. This is varified by comparison to other studies involving different geometries. The Nusselt number at reattachment seems to be dependent only on reattachment Reynolds number for these cases. The variation of the local Stanton number across the wake facing surface was found to be a function of angle of attack.