Heat Transfer In Cavity Research Articles

Numerical Study of Natural Convective Heat Transfer in a Two-Dimensional Cavity With Centrally Located Partition Utilizing Nanofluids

A two-dimensional single-phase natural convective heat transfer in a cavity with centrally located thin partition utilizing nanofluids has been numerically analyzed. The nanofluid used, which is composed of aluminum nanoparticles in suspension of Benzene, was provided at various solid volume fractions. The study is carried out numerically for a range of Rayleigh numbers, solid volume fractions, partition heights, and aspect ratios. Regions with the same velocity and temperature distributions are identified as a symmetry of sections. One-half of such a rectangular region is chosen as the computational domain, taking into account the symmetry about the thin partition. The governing equations are modeled by a stream function-vorticity formulation and are solved numerically by finite-difference schemes. Comparison with previously published numerical and experimental results showed excellent agreement. It is demonstrated that the partition height has a strong effect on both the heat transfer rate and the flow pattern. Results are presented in the form of streamlines and isotherm plots. The variation in the local Nusselt number along the thin partition provides valuable insight into the physical processes. A new correlation is proposed for the heat transfer studies in a wide range of thermal and geometric parameters.

Pulsating flow and convective heat transfer in a cavity with inlet and outlet sections

This paper deals with the study of 2-D, laminar, pulsating flow inside a heated rectangular cavity with different aspect ratios. The cooling liquid (water with temperature dependent viscosity and thermal conductivity) comes and leaves the cavity via inlet and outlet ports. The flow topology is characterised by the large recirculation regions that exist at inner corners of the cavity. These low velocity regions cause the heat transfer to be small when compared, for instance, to that of a straight channel. We study the effect that a prescribed pulsation at the inlet port has on the cavity heat transfer. This pulsating boundary condition, of the unsteady Poiseuille type, is described by its frequency and the amplitude of the pressure gradient. The time averaged Reynolds number of the flow, based on the hydraulic diameter of the inlet channel, is 100 and we consider that the dimensionless pulsation frequency (Strouhal number) varies in the range from 0.0 to 0.4. We show that the prescribed pulsation enhances heat transfer in the cavity and that the mechanism that causes this enhancement appears to be the periodic change in the recirculation flow pattern generated by the pulsation. Regarding the quantitative extent of heat transfer recovery, we find that appropriate selection of the pulsation parameters allows for the cavity to behave like a straight channel that is the configuration with the highest Nusselt number.

Thermal performance of a cubic cavity with a solar control coating deposited to a vertical semitransparent wall

We present a theoretical and experimental study of combined heat transfer in a cubic cavity containing non-participating air. The cubic cavity has a vertical semitransparent wall (glazing) with a solar control coating (SCC); an opaque black isothermal wall forms its opposite side. The top, bottom and side walls are opaque, gray and adiabatic. In the theoretical study, the 3-D steady state conservation equations for the mass, momentum and energy, along with the coupled radiation and conduction equations, were solved numerically by the finite volume method. The conduction for the semitransparent wall and the radiative energy flux were coupled through their boundary conditions at the convection model. Also, the semitransparent wall with SCC exchanges heat by convection and radiation to the exterior of the cavity. In the experimental study, the solar absorptance of the SCC was simulated experimentally using a thin film electrical resistance located on the glazing surface. Infrared imaging thermography was used to measure the temperature of the exterior surface temperature of the glazing. The interior air temperatures of the cavity were measured using thermocouples. The measured exterior surface temperatures of the glazing were introduced into the theoretical model as a boundary condition and the temperatures of the air at the interior of the cavity were compared with the theoretical ones predicted from the computational code for Ra = 2.3 × 10 6. Their average difference was 1.86%. Through these results, detailed descriptions of the air flow and temperature profiles in the cubic cavity are presented. The influence of radiative process on the overall heat transfer in the cavity is given particular attention, thus distinguishing the convective and radiative heat transfer in the cavity was shown separately. A parametric study was carried out for SCC absorptances of 0.08, 0.50 and 0.64 and exterior temperatures of 15 °C, 25 °C and 30 °C. It was found that for an exterior temperature of 25 °C, the radiative heat flux increases as the absorptance of the SCC increases from 0.08 to 0.64, but the solar heat gain coefficient (SHGC) decreases from 0.94 to 0.52. A new correlation for the Nusselt number as a function of the SCC absorptance is introduced as Nu = 0.9525 α + 10.985 for an ambient temperature of 25 °C.

The effect of rotating cylinder on the heat transfer in a square cavity filled with porous medium

The heat transfer in a square cavity filled with clear fluid or porous medium is numerically investigated in the present study. To change the heat transfer in the cavity a rotating circular cylinder is placed at the centre of the cavity. The ratio of cylinder diameter to cavity height is chosen as 0.8. Depending on the angular velocity of the cylinder the convection phenomena inside the cavity becomes natural, mixed, and forced. To keep the number of data low the Grashof number, Gr, is set to 10 6, while the parameter defining the convection regime in the cavity, Gr/ Re 2, is changing from 0.0625 to 10 2. The Darcy number in the cavity is set to 10 −2, 10 −3, and 10 −4. Galerkin finite element method is used to solve the Navier–Stokes equations with Brinkman–Forcheimer extended Darcy’s law, and energy equation in 2-D non-dimensional form. The solution methodology is compared and validated with the literature for a similar problem, and good agreement is achieved. The results are presented in terms of Nusselt numbers, velocity profiles and temperature contours. The results show that rotation is more effective in the forced convection regime than in mixed and natural convection regimes, and at high spin velocities the heat transfer is almost independent from the Darcy number.

Localized Heat Transfer in Buoyancy Driven Convection in Open Cavities

Background. An experimental study of buoyancy driven convection heat transfer in an open cavity was conducted. Method of Approach. Test cavities were constructed with calorimeter plates bonded to Styrofoam insulation. The inside of the cavities was heated and then exposed to ambient air for approximately thirty minutes. Different size cavities were examined at inclination angles of 0, 45, and 90deg. The heat transfer coefficient was determined from an energy balance on each calorimeter plate. The cavity’s plate temperatures varied spatially due to the transient nature of the tests. A parameter describing the nonisothermal cavity wall temperature variation was defined in order to compare with isothermal cavity heat transfer results. Results. Results showed that the cavity Nusselt number, based on a cavity averaged temperature, was insensitive to the transient development of nonisothermal conditions within the cavity. Comparison of cavity-average Nusselt number for the current study, where the Rayleigh number ranged from 5×106 to 2×108, to data from the literature showed good agreement. Cavity-average Nusselt number relations for inclination angles of 0, 45, and 90deg in the form of NuH,cav=CRa1∕3 resulted in coefficients of 0.091, 0.105, 0.093, respectively. The 45deg inclination angle orientation yielded the largest Nusselt numbers, which was similar to previous literature results. Trends in the local plate Nusselt numbers were examined and found similar to data from the literature.

Recent developments in gas turbine component temperature prediction methods, using computational fluid dynamics and optimization tools, in conjunction with more conventional finite element analysis techniques

Accurate temperature predictions are essential to the optimization of gas turbine component design. This paper provides an update on the application of recent developments in heat transfer boundary condition derivation and finite element model validation. Computational fluid dynamics (CFD) is increasingly being used to determine cooling flow distributions and convective heat fluxes on a range of components. Validation of the CFD methodology for internal cavity heat transfer is also a key focus of major research programmes. In this paper, further results are presented for selected engine and rig cavities. A fully coupled CFD/finite element thermal model solution is also demonstrated. Increasingly, the application of optimization techniques to the thermal model calibration process is showing that significant savings in analysis time can be achieved for a given accuracy of ‘match’. The optimization process is described and sample results are presented from the calibration of a typical thermal model. Finally, the impact of these new analysis techniques on the derivation of thermal boundary conditions in gas turbine component cavities and the implications for compliance with Airworthiness Authority regulations are summarized with respect to offering an improved temperature prediction validation strategy.

Flow and heat transfer in cavities between rotor and stator disks

The paper discusses the aerodynamic effects in a rotor–stator arrangement where rotor-induced air flow is important in the gap between rotor and housing bases. The rotor is adiabatic whereas the housing surfaces are assumed isothermal. For the case of a closed housing, the empirical correlations available in the literature for the estimate of moment coefficients due to aerodynamic effects under similar geometrical and kinematic conditions are compared with the results of CFD simulations. The numerical results on moment coefficients, mechanical power dissipation, and velocity fields are in satisfactory agreement. In order to evaluate temperature fields and heat fluxes, because no empirical correlations were found for the adiabatic-rotor/isothermal-stator conditions of interest, a semi-empirical model was developed, based on mass and angular momentum balances and the Reynolds analogy. Numerical results and approximate estimates of temperature distribution on the rotor surfaces are in reasonable agreement, also for the case of a housing open to radial flow through the gap.

Advances in modelling heat transfer through wood framed walls in fire

AbstractDescribed in this paper are advances made in modelling heat transfer through wood framed walls in fire. Previously unpublished experimental results are also given. This type of modelling is used for the determination of the performance of fire safety systems, such as wood framed wall barriers, in accordance with new performance‐based building regulations being introduced around the world. Advances include a discrete modelling method for radiative heat transfer in cavities with re‐entrant corners and gaps formed by the shrinkage of stud cross‐sections. It has been shown that the formation of the gaps can prevent temperatures rising in the fire side of studs by as much as 100–200°C. A simple means of modelling heat transfer by the movement of moisture and vapour, involving the modification of conductivity values has been developed. Sloughing of gypsum board sheets has been satisfactorily modelled assuming that a sheet sloughs when the temperature on the surface opposite the fire reaches the melting point of glass fibres in the gypsum board; that is, approximately 700°C. Recommendations on thermal properties obtained independently by other researchers are presented. Overall, the advances improve temperature predictions and broaden the range of walls that can be modelled including staggered stud walls as well as ordinary cavity walls. Copyright © 2002 John Wiley & Sons, Ltd.

Thermal properties of a variable cavity wall

An experimental investigation of heat transfer through a variable aspect ratio cavity wall has been conducted with the aid of a guarded hot box. A block and brick cavity wall measuring 1.2×1.2 m 2 was tested at cavity depths of 78, 60 and 40 mm. The influence of cavity flow upon the rate of heat transfer was established by correlation of convective cavity flows, measured by means of laser Doppler anemometry and by measuring surface temperatures on the four cavity faces. The experimental results show that with increasing aspect ratio, flow magnitude falls, circulation intensity reduces and the thermal resistance of the air cavity is thereby increased. Computational predictions were also used for the purpose of gaining an insight into cavity heat transfer and extrapolating models for further configurations.

Three‐dimensional simulation of natural convection in cavities with side opening

Naturally buoyant flow and heat transfer in a cubic cavity with side opening is analyzed numerically. Vertical wall of the cavity is at a higher temperature than the ambient, while other walls are assumed to be adiabatic. Numerically accurate results are presented for Rayleigh numbers of 103 to 106 for a fluid having a Prandtl number of 0.71. Unstable flow is predicted for Ra = 1 × 107. The aim of the work is twofold; studying three‐ dimensional flow and heat transfer in a cavity and comparing three‐dimensional results with two‐dimensional approximation to verify the validity of the two‐dimensional model. Convection fluxes are calculated using QUICK scheme with ULTRA‐SHARP flux limiter for two and three‐dimensional simulation. The results indicate that as Rayleigh number increases the difference between two and three dimensional predictions increases. Also, it is found that this difference is greater for the flow field than for the rate of heat transfer.

Power handling capabilities of superconducting YBCO thin films: Thermally induced nonlinearity effects

We have measured the microwave power dependence of the surface impedance Zs of YBa2Cu3Ox thin films up to very high microwave power levels. Films with different crystal qualities, including one with a bicrystal Josephson junction, were investigated. The experiments included both frequency-domain and pulsed time-domain measurements using a 14 GHz TE011 dielectric cavity. Our results demonstrate that the dissipation of heat, generated by rf currents in the superconducting film, contributes to the observed nonlinearities in the surface resistance. The relative extent of this contribution is determined primarily by the film quality. A simple Fabry-Perot resonator model, combined with a cavity heat transfer model, was used to analyze the effects of such nonlinearities on the electromagnetic response of the dielectric cavity to a pulsed input signal.

FREE SURFACE FLOW AND HEAT TRANSFER IN CAVITIES: THE SEA ALGORITHM

This study concerns the mathematical modeling of heat transfer and free surface motion under gravity, in cavities partially filled with a liquid. This two-phase flow problem is solved using a single-phase technique that assumes the air and liquid occupying the volume of the cavity can be treated as a single fluid with a sharp property discontinuity at the interface. A two-valued scalar advection equation is solved to mark the extent of each fluid. This idea is simple in concept, but requires careful application for two reasons: (1) The interface must remain sharp throughout the simulation; and (2) the equations of motion have to be expressed in a way that prevents the numerical “smothering” of the lighter fluid by the heavy one during the iteration process. To satisfy (1), the Van Leer TVD differencing scheme is adopted for the scalar advection equation, with appropriate flux corrections in the momentum and enthalpy equations. To satisfy (2), the continuity equation is expressed in volumetric form. The technique is incorporated in the scalar equation algorithm (SEA), It is applied to two problems, the first being the collapse of a liquid column in a sealed cavity with wall heat transfer, and the second the filling and simultaneous cooling of a mold with liquid aluminum.

Disk heat transfer in a rotating cavity with an axial throughflow of cooling air

A heated, rotating cavity with an axial throughflow of air is used as an experimental model for a pair of gas turbine, high-pressure compressor disks. Tests were carried out on a single rotating cavity comprising two disks of radius b = 0.4845m, bounded at the circumference by a carbon fiber shroud. Experiments were conducted with and without a heated shroud and for the range of parameters: 0.03 ≤ βΔT ≤ 0.3; 2 × 10 3 ≤ Re z ≤ 16 × 10 4 and 2 × 10 5 ≤ Re φ ≤ 5 × 10 6; for cavity gap ratios G = 0.13 and 0.36, and a constant value of inlet radius ratio of a/ b = 0.1. Measurements were also made of the air temperature inside the cavity by three thermocouple probes. The heat transfer from the disks was measured using thermopile flux meters. The measurements of cavity air temperature and cavity heat transfer were used to estimate the fraction of the central throughflow entering the cavity. This shows only a slight dependence on the gap ratio. For Ro < 1, approximately 50 percent is found to enter the cavity; for Ro > 10, this decreases to around 10 percent. Two mechanisms appear to operate in influencing the heat transfer. Firstly, heating of the air inside the cavity destablizes it and convection occurs under the action of rotationally induced buoyancy forces. Secondly, the central throughflow encourages mixing with the cavity air, which can either affect the heat transfer directly (as, for example, at the inner radii of the disks) or indirectly through the action of vortex breakdown. For the smaller gap ratio cavity, rotational effects become increasingly important toward the outer radius across a wide range of Rossby numbers. For the wider gap ratio cavity, this is restricted to a narrower range of Ro. In the region 4 ≤ Ro ≤ 5, the gap ratio plays a crucial role in affecting the disk heat transfer. At smaller values of Ro, where a significant fraction of the throughflow enters the cavity, the disk heat transfer rate does not appear to be affected by the gap ratio. At larger values, increasing the gap ratio also increases the heat transfer.

Prandtl number effects on laminar mixed convection heat transfer in a lid-driven cavity

This paper considers the flow and heat transfer in a square cavity where the flow is induced by a shear force resulting from the motion of the upper lid combined with buoyancy force due to bottom heating. The work is motivated by the application in the production of plane glass where the glass sheet is pulled over a bath of molten metal while being cooled and solidified. The numerical simulations, therefore, are performed for two-dimensional laminar flow (100 ⩽ Re ⩽ 2200), and effects of small to moderate Prandtl numbers (i.e. 0.01 < Pr < 50) on the flow and the heat transfer in the cavity are investigated for different values of Richardson number. The temperature and the flow fields in the cavity are calculated and presented to illustrate the strong influence of Prandtl number. The local and average Nusselt numbers are also reported for different values of Re, Ri, and Pr.

The effect of internal heat transfer in cavities on the overall thermal conductivity

Based on an analytical model, degradation of the overall thermal conductivity is studied in this paper to account for the internal mechanism of heat transfer between the solid medium and the fluid inside cavities. The cavity is assumed to randomly distribute in the solid with its density measured by the volume fraction. The model is capable of characterizing the effect of internal heat transfer such as conductive and convective heat transfer across the surfaces of cavities. In the case involving the convection mode, the Biot number has been identified to be the modulus dominating the overall thermal conductivity, while in the case involving the conduction mode, the ratio of the thermal conductivity of the solid medium to that of the fluid inside cavities is the major influencing factor. The effect of interfacial conduction and convection is found to be pronounced when the volume fraction of pores in the solid exceeds 15%. The large deviation from the experimental result by considering insulated cavities is redeemed by considering heat balance of heat conduction or convection across the cavity surface.

Effects of bottom injection on heat transfer and fluid flow in rectangular cavities

A numerical study is performed for investigating the effects of bottom injection on the heat transfer in a cavity. The Reynolds number (based on the cavity height and bulk mean velocity over the cavity) ranges in value from 100 to 1000. The cavity aspect ratios W/D investigated are 1, 6, and 10. The injection flow rate varies between 2-20% of the bulk flow rate. With the injectant temperature maintained the same as the cavity wall temperature, the effects of injection-reduced cavity heat transfer are strongly dependent on all the parameters aforementioned. For given injection rate and Reynolds number, the degree of heat-transfer reduction increases with a decrease of cavity aspect ratio. In addition, the current results suggest that the heat-transfer correlations presented in previous studies for cavities with aspect ratio of unity overestimate the extent of heat-transfer reduction for wider cavities.

Cavity Heat Transfer on a Transverse Grooved Wall in a Narrow Flow Channel

Measurements are presented of local convection heat transfer for the case of flow through a narrow slot-type channel where one of the bounding walls contains a transverse rectangular cavity. The experimental situation is a stationary modeling of some salient features of flow through the clearance gap at the grooved tips of axial turbine blades. Cavity depth-to-width ratios of 0.1, 0.2, and 0.5 are included for each of clearance-to-width ratios of 0.05, 0.10, and 0.15. Overall heat transfer on the cavity floor is in general reduced as cavity depth is increased, but reduction with the deepest cavity tested is essentially the same as that of the intermediate depth cavity. Resistance to flow through the gap is increased as cavity depth is increased, but again the change between the deepest and intermediate depth cavities is small. In addition to the stationary experiments, heat transfer in the cavity with a moving as well as stationary shroud is modeled with a finite-difference method. The numerical results indicate that, within the range of parameters considered, heat transfer characteristics in the cavity are virtually unaffected by the shroud movement. This is in agreement with a previous finding for heat transfer on ungrooved blade tips.

Mixed, Forced and Natural Convective Heat Transfer in Cavities Heated from Bottom Surface

This experimental study concerns the mixed, forced and natural convection heat transfers in rectangular cross-section cavities heated from bottom surface. Flow visualization, measurements of temperature profiles and heat transfer from heating surface are performed for various free-stream velocities, temperature differences between the free stream and the heating surface and dimensions of the cavity. It is found that flow pattern and the amount of convective heat transfer are influenced by an interaction between forced and natural convections, and there exists a range of Reynolds numbers in which the amount of convective heat transfer decreases with an increasing Reynolds number. this paper correlates the experimental data in terms of Nusselt number, dimension ratio of cavity (depth/width of cavity), Grashof number and Reynolds number with the range of Reynolds numbers.

Mixed, forced and natural convective heat transfer in cavities heated from bottom surface.

This experimental study concerns the mixed, forced and natural convection heat transfer in rectangular cross-section cavities heated from bottom surface. Flow visualization, temperature profiles and measurements of heat transfer from bottom heating surface are performed for various free-stream velocities, temperature differences between the free-stream and the heating surface and dimensions of the cavity. It is found that flow pattern and amount of convective heat transfer are influenced by an interaction between forced and natural convection, and there is an existence of Reynolds number's region which the amount of convective heat transfer is decreased with increasing Reynolds number. This paper correlates the experimental data in terms of Nusselt number, dimension ratio of cavity (depth/width of cavity), Grashof and Reynolds numbers according to the region of Reynolds numbers.

Heat transfer by conduction, natural convection and radiation across a rectangular cellular structure

This paper describes results on the effects of wall conduction and radiation heat exchange among surfaces on laminar natural convection heat transfer in a two-dimensional rectangular cavity modelling a cellular structure. Parametric heat transfer calculations have been performed, and numerical results are presented in graphical and tabular form. Local and average Nusselt numbers along the cavity walls are reported for a range of parameters of physical interest. The findings suggest that the local or the average Nusselt number is one of many parameters that control conjugate heat transfer problems. The results indicate that natural convection heat transfer in the cavity is reduced by heat conduction in the walls and radiation exchange among surfaces. The results obtaibed for the total heat transfer rate through the system using the two-dimensional model are compared with those based on a one-dimensional model.