Flow Heat Transfer Rate Research Articles

Estimation of Turbulent Heat Transfer Rates for Flow Over Buildings Using Openfoam

Formation of Urban Heat Islands is one of the serious issues faced by the cities in India. Due to anthropogenic activities there are abnormal variations in temperature both in day and night in cities as compared to rural areas. When we consider the governing equations the heating coefficients present, which depend on the buildings and local flow, can accelerate or decelerate the effect of temperature in the urban areas. In this paper the coefficient called eddy diffusivity of heat and its influence around high-rise buildings is analyzed using Computational Fluid Dynamics simulations with the OpenFOAM software package.

Finite-amplitude instability of magnetohydrodynamic mixed convection flow of liquid metals in a vertical channel

The study of mixed convective liquid metal flow under a magnetic field has good potential for designing the next generation of advanced magnetohydrodynamic (MHD) devices. Flow instabilities play a crucial role in understanding flow dynamics in different applications. This paper reports the finite-amplitude instability of MHD mixed convection flow of electrically conducting liquid metals in a vertical channel under a transverse magnetic field. The cubic Landau equation is derived to determine the subcritical/supercritical bifurcations in the flow. The weakly nonlinear stability results are examined for a good range of Ha and Pr in the vicinity and far from the linear stability critical point (bifurcation point) at Reynolds number Re = 5000. The finite-amplitude results give only the supercritical type of instability for the considered range of Ha and Pr. The equilibrium amplitude increases by increasing the strength of the applied magnetic field. The impact of the nonlinear interaction of different harmonic modes on friction coefficient, heat transfer rate, and wave speed is also examined for the linearly unstable region. The results show that the distorted mean flow's heat transfer rate is higher than that calculated by the laminar basic state. Like linear stability analysis, the nonlinear analysis shows that the applied magnetic field stabilizes the basic flow. The supercritical instability shows that the linearly unstable flow shows a smooth transition.

MULTIPHYSICAL MATHEMATICAL MODEL OF HEAT AND MASS EXCHANGE IN RECEIVER AND THERMAL STORAGE SYSTEM OF THE SOLAR THERMODYNAMIC PLANT

The main stages of creation of the multiphysical mathematical model of heat exchange in receiver system of the solar thermodynamic system with concentrator are presented in this study. The statistic approach for modeling of the heat fluxes in system «concentrator – heat receiver» is used in first stage. Obtained distribution of concentrated solar flow at outer heat receiver surface is applied as boundary condition for numerical modeling of the convective heat exchange in the heat transfer fluid. The average heat transfer fluid temperature is found in the second stage research. This temperature is determined type of the thermal storage material for the thermal storage module with phase change «solid - liquid». Geometry of the thermal storage module, temperature fields in the thermal storage material, heat transfer flow rate, which ensures given input temperature in energy conversion system were determined.

Selective exhaust gas recirculation in combined cycle gas turbine power plants with post-combustion CO2 capture

In the context of the process intensification of Post-combustion Carbon Capture (PCC), Selective Exhaust Gas Recirculation (SEGR) in Natural Gas-fired Combined Cycle (NGCC) plants, a concept where CO2 is selectively recycled to increase concentration and reduce flow rates of the flue gas, is analysed. SEGR operated either in parallel or in series with a downstream PCC system increases CO2 concentration beyond 14 vol% and maintains oxygen levels in the combustor to approximately 19 vol%, well above the 16 vol% limit reported for non-selective Exhaust Gas Recirculation (EGR). Process modelling shows that the current class of gas turbine engines can operate without a significant deviation in the compressor and in the turbine performance. Compressor inlet temperature and CO2 concentration in the working fluid are the two critical parameters affecting the gas turbine net power output and efficiency. A higher turbine exhaust temperature allows the generation of additional steam in the HRSG. This results in an increase in net power output of approximately 42 MW (5.2%) and 18 MW (2.3%), and in net thermal efficiency of 0.55%point and 0.83%point, for the investigated configurations with SEGR in parallel and SEGR in series, respectively. With 30 wt% aqueous monoethanolamine scrubbing technology, SEGR leads to operation and cost benefits. SEGR in parallel with 70% recirculation, 97% selective CO2 transfer efficiency and 96% PCC efficiency results in a reduction of 46% in packing volume and 5% in specific reboiler duty, compared to air-based combustion CCGT with PCC, and of 10% in packing volume and 2% in specific reboiler duty, compared to 35% EGR. SEGR in series operating at 95% selective CO2 transfer efficiency and 32% PCC efficiency results in a reduction of 64% in packing volume and 7% in specific reboiler duty, compared to air-based CCGT with PCC, and of 40% in packing volume and 4% in specific reboiler duty, compared to 35% EGR. On selecting a technology for SEGR applications, CO2 selectivity, pressure drop and heat transfer flow rate are the operating parameters with a larger effect on the power plant performance with SEGR. It is important to minimise oxygen leakages from the air into the flue gas, minimise heat transfer that would otherwise increase CO2-enriched air temperature at compressor inlet, and minimise pressure drop, e.g. a 1 kPa pressure drop results in gas turbine derating of 2 MW (0.7%).

Visualization of flow patterns and bubble behavior during flow boiling in open microchannels

Flow boiling in microchannels is favored by the heat transfer community due to the high heat transfer rates that can be obtained with lower mass flow rates. However, the heat transfer rates for flow boiling in microchannels are impacted by flow reversals and flow instabilities. An open microchannel structure was recently proposed to reduce the impact of the flow boiling instabilities. Subcooled flow boiling experiments were conducted in open microchannels using deionized water. The open microchannels had 6 parallel channels with a 0.3mm uniform thickness gap above them The channels were fabricated on a 6mm×40mm copper block. The channels were 0.5mm wide and 0.3mm deep with 0.43mm wide fins between them. Flow visualizations were performed with a high-speed CCD camera with the results showing that the flow regimes in the open microchannels differ from those in closed microchannels with stratified flow and no flow instability. Two types of confined bubbles were observed with characterizations of the effects of the bubbles on each other. The heat transfer mechanisms for flow boiling in open microchannels are also described.

Open and closed metal hydride system for high thermal power applications: Preheating vehicle components

Many vehicle components operate at temperatures above ambient conditions. At cold start, most of the pollutants are produced and lifespan is reduced. Thermochemical energy storage with high power density could prevent these disadvantages. In order to investigate achievable power densities of a thermochemical energy storage at technically relevant boundary conditions, a laboratory scale device using metal hydrides (LaNi4.85Al0.15 and C5®) is designed and preheating operation modes (open and closed) are analyzed. The impact of the ambient temperature (from −20 to +20 °C), a s well as other influencing factors on the thermal power output such as heat transfer flow rate, regeneration temperature and pressure conditions are investigated. The experiments proved the suitability of the reactor design and material selection for the considered application boundary conditions. For the coupled reaction (closed system), the ambient temperature has the greatest influence on the thermal power with decreasing values for lower temperatures. Here, values between 0.6 kW/kgMH at ambient temperature of −20 °C and 1.6 kW/kgMH at 20 °C, at otherwise same conditions, were reached. If hydrogen can be supplied from a pressure tank (open system), the supply pressure in relation to equilibrium pressure at the considered ambient temperature has to be large enough for high thermal power. At −20 °C, 1.4 kW/kgMH at a supply pressure of 1.5 bar and 5.4 kW/kgMH at a hydrogen pressure of 10 bar were reached.

The study of entropy generation during flow boiling in a micro-fin tube

Increasing in the heat transfer rate in flow boiling is a common and key issue for engineers. Generally, the heat transfer coefficient augmentation methods are divided into two main categories (active and passive methods). In passive methods the increase in heat transfer rate causes the increase in pressure drop. In order to evaluate the contribution of heat transfer and pressure drop mechanisms, the entropy generation analysis is used. In this paper, the entropy generation in micro-fin tube is investigated under flow boiling condition. The effect of different geometrical parameters and flow conditions is discussed on pressure drop contribution and heat transfer one in entropy generation, irreversibility distribution ratio (IDR) and Bejan number (Be). The frictional pressure drop and heat transfer coefficient in the micro-fin tube and the helically coiled one are compared as two enhancements passive heat transfer methods with the smooth straight tube in the literatures. Therefore, by introducing entropy generation number (Ns), the favorable geometry between the micro-fin tube and the helically coiled one with respect to the smooth straight tube is recognizable at equivalent boundary conditions.

Investigation of entropy generation in a helically coiled tube in flow boiling condition under a constant heat flux

There are many methods to augment the heat transfer rate in flow boiling in industrial applications. The helically coiled tubes are one of the best geometries to enhance the heat transfer rate. The entropy generation analysis is an appropriate tool to evaluate the contribution of heat transfer and pressure drop mechanisms. In the present paper, the entropy generation in the helically coiled tube under flow boiling is studied. The optimum tube and coil diameters under specified conditions are found. The effect of different flow conditions such as mass velocity, inlet vapor quality, saturation temperature, and heat flux on contributions of pressure drop and heat transfer in entropy generation is discussed. The Bejan number (Be) and irreversibility distribution ratio (IDR) at different saturation temperatures versus mass velocity are plotted. The comparison between entropy generation and contributions of pressure drop and heat transfer for the helically coiled tube and the straight one is presented. The entropy generation number (Ns) for different flow conditions is plotted. The entropy generation analysis shows that there is a favorable region to use the helically coiled tube with respect to the straight one.

Flow and heat transfer measurements in a planar offset attaching jet with a co-flowing wall jet

Measurements were performed to characterize the flow, mean skin friction, and heat transfer rate in flows consisting of a turbulent planar jet with Reynolds number of approximately 42,000 offset one jet height above a wall and a co-flowing wall jet with an initial height 0.18 times the offset distance (Hs). The results were compared to previous measurements for flows with similar offset jets and a co-flowing wall jet with initial height 0.5 times the offset distance. Low velocity wall jets in both geometries were quickly entrained into the offset jet increasing the attachment length of the offset jet and reducing the turbulent fluctuations and the heat transfer rate in the near field of the attaching jet. These changes increased with the mass flow rate of the wall jet as less mass is recirculated upstream from the attachment region. Higher velocity wall jets in both geometries initially remained attached to the wall causing a recirculation region to form between the jets that induce periodic motions that significantly increase the wall normal turbulent fluctuations in the flow. The periodic motions in the flows with the smaller wall jets significantly increased the pressure fluctuations on the wall below the near field flow and enhanced the heat transfer in this region. This differed from the results for the offset jet with the larger co-flowing wall jet where the flow fluctuations did not have a significant effect on the wall pressure fluctuations and the heat transfer below the flow. The difference in the effect of the periodic motions for flows appears to be due to differences in the forcing of the offset jet by the periodic motions and differences in the interaction between the recirculation region and the wall due to the height and mass flux of the wall jets.

Measured Pressure Distribution and Local Heat Transfer Rates for Flow Over Concave Hemispheres

The use of parachutes for recovery of equipment from very high velocity vehicles, such as missiles, has directed attention to the need for aerodynamic and heat transfer information associated with such high drag bodies. With this application in mind, a solid concave hemisphere, positioned in a subsonic flow, was studied. Plexiglas models were placed in water and in air streams, and by using straightforward flow visualization techniques it was determined that the flow in the cup was, in essence, a separate flow region. Quantitative measurements were made of the pressure distribu­ tion and local heat transfer rate over a Reynolds number range of 6000 to 89,000. The pressure on the inner surface of the hemisphere was found to be equal to the total pressure from the stagnation point to an angle of 75 deg, dropping somewhat at greater angles. The heat transfer was found to increase monotonically from the stagnation point to the edge. The overall dimensionless heat transfer may be given in the form NuD/(ReD)^2 = 0.42. R ECENT interest in the use of parachutes for the re­ covery of equipment from high speed vehicles has di­ rected attention to the aerodynamics and heat transfer of extremely high drag bodies. At the time the present study of heat transfer to parachutes was initiated, only limited aerodynamic information was available, and no pertinent heat transfer information was found in the open literature. [Recently, information on the pressure distribution about a hemisphere with 20 per cent porosity (l)3 and heat transfer data for a fist-type parachute (2) has become available.] To obtain preliminary estimates of the time-temperature history of descending parachutes, the porosity was neglected and the convective heat transfer coefficients were estimated for an upstream facing hemisphere. The convective coeffi­ cient on the rear surface was assumed to be the same as on the rear surface of a sphere. For the front surface of the hemisphere, the potential flow solution of Schiffman and Spencer (3) yielded the velocity and pressure distribution, which, in turn, was used to determine the local convective heat transfer performance. The velocity and heat transfer results are shown in Fig. 1 and compared with established values for flow over the front side of a sphere; surprisingly, the predicted velocity and heat transfer coefficient distribu­ tion for the front face of the hemisphere is very similar to the distribution on the front face of a sphere. However, the validity of these results was brought sharply into question when preliminary pressure distribution measurements using a solid hemisphere gave markedly different behavior. As a consequence, a rather detailed aerodynamic and heat transfer investigation using concave hemispheres was initiated.