Reduction In Heat Transfer Rate Research Articles

Analysis of natural convection in a square cavity with a thin partition for linearly heated side walls

Purpose – The purpose of this paper is to optimize the heat transfer rate in square cavity by attaching fin at the bottom wall. Design/methodology/approach – The problem is formulated and solved using finite element method. Accuracy of the method is validated by comparisons with previously published work. Findings – It was found that attaching fin reduces heat transfer rate in the cavity. Originality/value – Although the problem is not very original it is important in that many applications have heating on adjacent walls.

Heat Transfer Measurements in Rotating Blade–Shape Serpentine Coolant Passage With Ribbed Walls at High Reynolds Numbers

Flow in the internal three-pass serpentine rib turbulated passages of an advanced high pressure rotor blade is simulated on a 1:1 scale in the laboratory. Tests to measure the effect of rotation on the Nusselt number are conducted at rotation numbers up to 0.4 and Reynolds numbers from 75,000 to 165,000. To achieve this similitude, pressurized Freon R134a vapor is utilized as the working fluid. Experimental heat transfer coefficient measurements are made using the copper-plate regional average method. Regional heat transfer coefficients are correlated with rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Strikingly, a significant deterioration in heat transfer is noticed in the “hub” region—between the radially inward second pass and the radially outward third pass. This heat transfer reduction is critical for turbine cooling designs.

Group analysis and numerical computation of magneto-convective non-Newtonian nanofluid slip flow from a permeable stretching sheet

Two-dimensional magnetohydrodynamic bound- ary layer flow of non-Newtonian power-law nanofluids past a linearly stretching sheet with a linear hydrody- namic slip boundary condition is investigated numeri- cally. The non-Newtonian nanofluid model incorporates the effects of Brownian motion and thermophoresis. Similarity transformations and corresponding similarity equations of the transport equations are derived via a linear group of transformations. The transformed equa- tions are solved numerically using Runge-Kutta-Fehl- berg fourth-fifth order numerical method available in the Maple 14 software for the influence of power-law (rhe- ological) index, Lewis number, Prandtl number, thermo- phoresis parameter, Brownian motion parameter, magnetic field parameter and linear momentum slip parameter. Validation is achieved with an optimized Nakamura implicit finite difference algorithm (NANO- NAK). Representative results for the dimensionless axial velocity, temperature and concentration profiles have been presented graphically. The present results of skin friction factor and reduced heat transfer rate are also compared with the published results for several special cases of the model and found to be in close agreement. The study has applications in electromagnetic nano- materials processing.

Scaling of Convective Heat Transfer Enhancement Due to Flow Pulsation in an Axisymmetric Impinging Jet

Impinging jets are widely used to achieve a high local convective heat flux, with applications in high power density electronics and various other industrial fields. The heat transfer to steady impinging jets has been extensively researched, yet the understanding of pulsating impinging jets remains incomplete. Although some studies have shown a significant enhancement compared to steady jets, others have shown reductions in heat transfer rate, without consensus on the heat transfer mechanisms that determine this behavior. This study investigates the local convective heat transfer to a pulsating air jet from a long straight circular pipe nozzle impinging onto a smooth planar surface (nozzle-to-surface spacing 1 ≤ H/D ≤ 6, Reynolds numbers 6000 ≤ Re ≤ 14,000, pulsation frequency 9 Hz ≤ f ≤ 55Hz, Strouhal number 0.007 ≤ Sr = fD/Um ≤ 0.1). A different behavior is observed for the heat transfer enhancement in (i) the stagnation zone, (ii) the wall jet region and overall area average. Two different modified Strouhal numbers have been identified to scale the heat transfer enhancement in both regions: (i) Sr(H/D) and (ii) SrRe0.5. The average heat transfer rate increases by up to 75–85% for SrRe0.5 ≅ 8 (Sr = 0.1, Re = 6000), independent of nozzle-to-surface spacing. The stagnation heat transfer rate increases with nozzle-to-surface distance H/D. For H/D = 1 and low pulsation frequency (Sr < 0.025), a reduction in stagnation point heat transfer rate by 13% is observed, increasing to positive enhancements for Sr(H/D) > 0.1 up to a maximum enhancement of 48% at Sr(H/D) = 0.6.

Effects of solid particle properties on the thermal performance of a packed-bed molten-salt thermocline thermal storage system

In this study we present a transient two-dimensional dispersion–concentric (D–C) model for heat transfer and fluid dynamics in a packed-bed molten salt thermocline thermal storage system. Based on the developed model, the effects of solid particle properties including particle diameter and particle material on the heat transfer between molten salt and solid particles are investigated with emphasis on detailed temperature distributions within solid particles. The results show that heat transfer between solid particles and molten salt can be significantly influenced by particle diameter. Increasing the particle diameter reduces heat transfer rate between solid particles and molten salt, and finally results in a decrease in the effective discharging time and discharging efficiency. It is also found that the effect of solid material on the thermal performance is influenced by the particle diameter. When particle size is small enough, the efficiency of the thermocline system is nearly independent of the solid material. For large particle diameters, heat discharge rate from solid particles to molten salt may be reduced if the thermal conductivity of solid particle is too low.

Study of Thermal Response of a Skin Stimulant Material with a Protective Fabric under a Hot Air Jet

The present study focuses on the thermal response of a skin simulant material with a protective fabric (Kevlar 49) under a flame blast condition. This investigation has been conducted using a stream of hot air jet that has been impinged normally on the test surface with/without protective fabric to mimic the flame burst condition. The air jet temperature is 125 °C and the jet velocity is 15 m/s and 19 m/s. The temperatures at various radial positions of the solid are measured using thermocouples and are used to calculate the surface heat flux to the skin simulant material. Subsequently, the local heat transfer coefficient and the local Nusselt Number for different radial positions of the base plate have been estimated, illustrated and analyzed for different longitudinal distance between nozzle and the test setup. The experimental results are then compared with Stoll Second Degree Burn Criterion of the ASTM Standard (F1060-01). The results show a significant reduction in heat transfer rate and exhibit an adequate protection due to the use of the protective fabric.

A parametric study of oscillatory two-phase flows in a single turn Pulsating Heat Pipe using a non-isothermal vapor model

The capillary U-tube is the most fundamental geometrical configuration of a Pulsating Heat Pipe (PHP). In this paper, an improvised numerical model is proposed for the investigation of thermo-hydrodynamics of pulsating two phase flow in a capillary U-tube. The mass, momentum and energy equations are solved using an explicit finite difference scheme. In this model, the vapor is assumed non-isothermal by considering saturation temperature at the liquid vapor interface in calculating the phase change mass and heat transfer instead of the vapor temperature as considered in earlier isothermal models. Pressure loss at bends and capillary effects at meniscus are also considered in the numerical model. A detailed parametric study is conducted to understand the effects of pressure loss at bends, surface tension, PHP orientation, adiabatic length and type of working fluid on the oscillatory two phase flow in a PHP. The inclusion of pressure loss at the bend results in a reduction in the amplitude of oscillation and therefore a reduction in heat transfer rate. It is observed that the inclusion of capillary effects at the liquid meniscus affects the initial distribution of liquid slugs and vapor plugs and it doesn't have any significant effect on steady state PHP operation. The vertical mode of operation is found to show better heat transfer characteristics as compared to horizontal mode of operation. The length of the adiabatic section has a significant effect on the performance of the PHP. Thermal performance reduces considerably with an increase in the adiabatic length. The type of working fluid used in the PHP has also a profound effect on the PHP characteristics. Water and n-pentane show better heat transfer rate as compared to ethanol. The thermal resistance of the U-tube PHP is found to decrease continuously with an increase in the diameter of the capillary tube.

MHD free convective boundary layer flow of a nanofluid past a flat vertical plate with Newtonian heating boundary condition.

Steady two dimensional MHD laminar free convective boundary layer flows of an electrically conducting Newtonian nanofluid over a solid stationary vertical plate in a quiescent fluid taking into account the Newtonian heating boundary condition is investigated numerically. A magnetic field can be used to control the motion of an electrically conducting fluid in micro/nano scale systems used for transportation of fluid. The transport equations along with the boundary conditions are first converted into dimensionless form and then using linear group of transformations, the similarity governing equations are developed. The transformed equations are solved numerically using the Runge-Kutta-Fehlberg fourth-fifth order method with shooting technique. The effects of different controlling parameters, namely, Lewis number, Prandtl number, buoyancy ratio, thermophoresis, Brownian motion, magnetic field and Newtonian heating on the flow and heat transfer are investigated. The numerical results for the dimensionless axial velocity, temperature and nanoparticle volume fraction as well as the reduced Nusselt and Sherwood number have been presented graphically and discussed. It is found that the rate of heat and mass transfer increase as Newtonian heating parameter increases. The dimensionless velocity and temperature distributions increase with the increase of Newtonian heating parameter. The results of the reduced heat transfer rate is compared for convective heating boundary condition and found an excellent agreement.

Heat Transfer Reduction due to a Ceiling-Mounted Barrier in an Enclosure with Natural Convection

Effects of a ceiling-mounted barrier on natural convection heat transfer in a square cavity with differentially heated wall are numerically investigated. A limit case, in which the partition has small thickness and low thermal conductivity, is studied. The study is performed for nine different locations of barrier on the ceiling, two different lengths of barrier as 15 and 50% of the side wall, and Rayleigh numbers from 103 to 106. The vorticity and streamfunction approach is used to obtain velocity distribution, and the energy equation is solved to determine temperature field in the cavity. The variations of the local Nusselt number on the hot and cold walls and the change of mean Nusselt number with the location of barrier in the cavities with different Rayleigh numbers are presented. The obtained results show that a wall-mounted barrier can be used to reduce heat transfer rate through the cavity; however, its effectiveness depends on length and location of barrier and Rayleigh number.

Jet Impingement Heat Transfer from a Circular Cylinder Located Between Confining Walls

*The heat transfer characteristics of a circular cylinder (diameter d) located between confining walls (at varying separation distance H apart) exposed to slot jet of air (of width w) from a contoured nozzle, and at a range of spacing z from the jet exit, has been studied experimentally and computationally. The study focussed on Reynolds number Re in the range 1000 – 12000. The results reveal that while the slot jet impinging on a cylinder in confined space generally yields higher average heat transfer rates relative to the uniform cross flow case; these are however almost always found to yield lower heat transfer rates in comparison with corresponding slot jet impingement without confining walls. Furthermore, in the Reynolds number range studied, it was found that a dimensionless confinement spacing H/d = 10 exists which consistently exhibits a minimum in cylinder heat transfer rate over all the non-dimensional jet exit to cylinder spacing z/w and ratio of cylinder diameter to slot width d/w that were investigated. It was established that reduction in heat transfer for confinements H/d between 10 and 16 is due to the jet becoming unstable and thereby periodically flapping across the cylinder, switching between the confining walls, in tandem with vortex shedding from the cylinder. This reduces the effectiveness of impingement and hence reduces the heat transfer rates. It is therefore concluded that jet impingement does not always enhance heat transfer rate, but may in fact reduce it under conditions in which the impingement target is located in the midst of confining walls. It was observed that the average of all the Nusselt numbers in a confined space reduces by about 17% when compared with a non confined cylinder. In summary therefore, the influence of confining walls around the cylinder has an unfavourable impact on the flow and therefore the heat transfer rates with jet impingement. The reduction in heat transfer rates with confining walls implies that a jet impingement heat transfer system designed using data obtained for jet impingement on a non-confined cylinder could compromise the effectiveness of the system, and may have a large bearing on the quality, efficiency and productivity as well as on the health and safety aspects of products and processes.

Simulation of Entrainment Mass Transfer in Annular Two-Phase Flow Using the Physical Concept

AbstractMost of two-phase heat transfer analysis involve estimation of dry-out phenomenon in order to find whether it takes place and if so, at what location in the tube it will happen. Dry-out phenomenon considerably reduces heat transfer rate and it should be avoided as far as possible. Entrainment highly affects dry-out occurrence chance and location. Therefore a great attention should be paid on simulation of entrainment in order to have a good estimation of heat transfer and to safely design thermal systems.Although there are many models available for simulation of entrainment phenomenon, few are instrumentally employed in heat and fluid flow analyses, since most of those are empirical and limitedly can be applied due to the restricted conditions of experiments. There also are few theoretical approaches in the literature for simulating entrainment phenomenon by applying the physical potential of entrainment but some of those have a considerable deviation from the available empirical models. In the present study a conceptual approach is employed to simulate entrainment. In this regard the most important potential of this phenomenon, known as interfacial waves, are studied and modeled to find how they will lead to entrainment and how much the amount of entrainment is in vertical annular two-phase flows. An entrainment correlation is proposed at the end and will be compared with the available empirical ones for the verification objectives.

Effect of a supersonic counterflow jet on blunt body heat transfer rates for oncoming high enthalpy flow

Heat transfer reduction studies, in the presence of a supersonic counterflow jet from the stagnation point of a 60-deg apex angle blunt cone, are conducted in a free piston driven hypersonic shock tunnel, HST3, to verify the effectiveness of this technique for high enthalpy flows. For flow of Mach number 8 with stagnation enthalpy of 5 MJ/kg, it has been observed that the heat transfer rate at some location decreases initially with increase in the injection pressure ratio until the critical injection pressure ratio is reached. A forty-five percent reduction in heat transfer rate near the stagnation point has been measured for the critical injection pressure ratio, equal to 14.91. A further increase in the injection pressure ratio reduces the overall percentage heat transfer reduction. The dependence of fluidic spike length on percentage heat transfer reduction has been confirmed.

Phase Change Heat Transfer Enhancement Using Copper Porous Foam

A detailed experimental and analytical study has been performed to evaluate how copper porous foam (CPF) enhances the heat transfer performance in a cylindrical solid/liquid phase change thermal energy storage system. The CPF used in this study had a 95% porosity and the phase change material (PCM) was 99% pure eicosane. The PCM and CPF were contained in a vertical cylinder where the temperature at its radial boundary was held constant, allowing both inward freezing and melting of the PCM. Detailed quantitative time-dependent volumetric temperature distributions and melt/freeze front motion and shape data were obtained. As the material changed phase, a thermal resistance layer built up, resulting in a reduced heat transfer rate between the surface of the container and the phase change front. In the freezing analysis, we analytically determined the effective thermal conductivity of the combined PCM/CPF system and the results compared well to the experimental values. The CPF increased the effective thermal conductivity from 0.423W∕mKto3.06W∕mK. For the melting studies, we employed a heat transfer scaling analysis to model the system and develop heat transfer correlations. The scaling analysis predictions closely matched the experimental data of the solid/liquid interface position and Nusselt number.

On the heat transfer rate reduction of structural insulated panels (SIPs) outfitted with phase change materials (PCMs)

This paper presents a study on the integration of two building technologies into one new unified form for application in residential and small commercial and industrial buildings. Structural insulated panel (SIP) technology was utilized as a structural vehicle, and also for thermal insulation, and phase change materials (PCMs) provided distributed thermal mass. This new type of wall panel was termed phase change material structural insulated panel (PCMSIP). The research conducted during this study provided the foundations for the development of this type of thermally enhanced wall panels and evaluated their thermal performance, based on heat transfer rate reduction, under full weather conditions. On average, the peak heat flux reductions produced by the PCMSIPs in combination with 10% and 20% PCM were 37% and 62%, respectively. The average reductions in daily heat transfer across the PCMSIPs were 33% and 38% for concentrations of 10% and 20% PCM, respectively. The percent PCM concentration was based on the weight of the interior wallboard.

Reduction of the Volatile Matter Evolution Rate from a Plastic Pellet during Bubbling Fluidized Bed Pyrolysis by Using Porous Bed Material

AbstractRapid volatile matter evolution from high‐volatile fuels such as wastes and biomass is one of problems associated with fluidized bed incinerators and gasifiers. When volatile matter evolves rapidly in the vicinity of the fuel feed point, the mixing of volatile matter with reactant gas is poor, and therefore, unreacted volatile matter is expected to be released from the reactor. In the present work, reduction of the volatile matter evolution rate was attempted by employing porous solids as bed materials instead of nonporous sand. The effect of bed material on the onset of devolatilization was measured by use of a bench‐scale bubbling fluidized bed reactor. Volatile matter capture by the porous solids (capacitance effect) and the heat transfer rate within the bed, both of which affect volatile matter evolution rate, were also measured. Four types of porous solids, both with and without capacitance effect, were employed as the bed material. By employing porous solids without capacitance effect, the contributions of reduced heat transfer rate and capacitance effect to the delay of volatile matter evolution can be evaluated separately. For porous bed materials with a moderate capacitance effect (volatile matter capture of up to 20 %), the delay of the onset of devolatilization, which was measured by detecting the flame combustion of the volatile matter, was explained by the lower heat transfer between the fuel and bed. However, for a porous particle with high capacitance effect (volatile matter capture of 30 %), the capacitance effect also affected the delay of the onset of the flame combustion.

Crack-free densification of ceramics by laser surface treatment

Laser surface melting and re-solidification has been applied to eliminate pores and create homogeneous surfaces in refractory ceramics, improving the high-temperature degradation resistance of such materials. The major problem in laser surface treatment of ceramics is the formation of cracks during cooling. Efforts have been made to eliminate this problem by applying pre- and post-heating techniques, as well as dual laser beam systems. Even though such efforts have been partially successful, they lack practicability. This paper reports on the formation of fully densified and crack-free surfaces of refractory ceramics by melting and re-solidification using a single laser beam in a one-step process, without applying any pre- and post-heating techniques. Formation of cracks was avoided due to modification in the heat transfer conditions on the material surface, resulting in slower surface cooling rates. The reduction in heat transfer rates led to change in the thermal shock conditions to which the material surface was subjected, since a constant cooling rate was established during the transition from plastic to elastic behaviour of the treated surface.

Film condensation in presence of non-condensable gases over horizontal tubes with progressively increasing radius of curvature in the direction of gravity

A theoretical study is executed to investigate the simultaneous influences of reduction in heat transfer rate on account of condensation in presence of non-condensables and its augmentation by the geometry of an horizontal tube surface with increasing radius of curvature in the direction of gravity. The tube surface profile considered for the present work is an equiangular spiral described in a polar form as: R = ae mθ ( a and m being parametric constants). It is observed that a very small bulk concentration (even less than 1%) of the non-condensable gas reduces considerably the heat transfer coefficient. However, there is an enhancement in heat transfer coefficient for condensation over a polar tube surface, as compared to that over a circular tube surface. This enhancement in heat transfer coefficient, with an increase in the value of m (a surface profile parameter), in presence of non-condensables is more than the corresponding proportional enhancement in the same in absence of any non-condensable species.

Film cooling effectiveness on a large angle blunt cone flying at hypersonic speed

Effectiveness of film cooling technique to reduce convective heating rates for a large angle blunt cone flying at hypersonic Mach number and its effect on the aerodynamic characteristics is investigated experimentally by measuring surface heat-transfer rates and aerodynamic drag coefficient simultaneously. The test model is a 60° apex-angle blunt cone with an internally mounted accelerometer balance system for measuring aerodynamic drag and an array of surface mounted platinum thin film gauges for measuring heat-transfer rates. The coolant gas (air, carbon dioxide, and/or helium) is injected into the hypersonic flow at the nose of the test model. The experiments are performed at a flow free stream Mach number of 5.75 and 0° angle of attack for stagnation enthalpies of 1.16MJ∕kg and 1.6MJ∕kg with and without gas injection. About 30%–45% overall reduction in heat-transfer rates is observed with helium as coolant gas except at stagnation regions. With all other coolants, the reduction in surface heat-transfer rate is between 10%–25%. The aerodynamic drag coefficient is found to increase by 12% with helium injection whereas with other gases this increase is about 27%.

Heat gain reduction by means of thermoelectric roof solar collector

This paper presents a numerical investigation on attic heat gain reduction by using thermoelectric modules integrated in a conventional roof solar collector (RSC). This system, called thermoelectric roof solar collector (TE-RSC), is composed of a transparent glass, air gap, a copper plate, thermoelectric modules (TE) and rectangular fin heat sink. Due to the incident solar radiation, a temperature difference is created between the hot and cold sides of TE modules that generates a direct current. This current is used to drive a ventilating fan for cooling the TE-RSC and enhancing attic ventilation that reduces ceiling heat gain. The system performance was simulated using TRNSYS program with new TE and DC fan components developed by our team and compared to a common house. Simulation results using real house configuration showed that a TE-RSC unit of 0.0525 m 2 surface area can generate about 9 W under 972 W/m 2 global solar radiation and 35 °C ambient temperature. The induced air change varied between 20 and 40 and the corresponding ceiling heat transfer rate reduction is about 3–5 W/m 2. The annual electrical energy saving was about 362 kWh. Finally, economical calculations indicated that the payback period of the TE-RSC is 4.36 years and the internal rate of return is 22.05%.

Effect of radial fins on two-dimensional turbulent natural convection in a horizontal annulus

Turbulent natural convection between two horizontal concentric cylinders in presence of radial fins is studied numerically. Turbulence modeling was done using a standard k– ε model. The conservation equations of mass, momentum and energy along with modeled equations of turbulent kinetic energy and its dissipation rate were discretized using control-volume approach. Results are obtained in the form of streamline and temperature contours for a number of radial fins ranges from 2 to 12 which are attached to the inner cylinder. Two different configurations are investigated for the case of two and four radial fins to reveal the effect of fin height and fin configuration. The Rayleigh number considered in this study ranges from 10 6 to 10 9. Results obtained for local Nusselt number variation of the inner cylinder shows that fin arrangement has no significant effect on heat transfer and higher fin heights have a blocking effect on flow causing lower heat transfer rate. It is observed that there is a reduction of heat transfer rate in all of the configurations considered in this study as compared to the case of no fin at the same Rayleigh number. There is also an increase in Nusselt number prediction with increasing Rayleigh number, which is a general behavior for all of natural convection problems.