Unheated Plate Research Articles

Numerical investigation on mixed convection with nanofluids in vertical channels with opposing moving plate

Abstract A numerical investigation on 2-dimensional steady-state mixed convection in a vertical channel asymmetrically heated with water-alumina (Al2O3) nanofluids is performed to evaluate the thermal and fluid dynamic behavior. The interest in this problem is related to several technological applications such as nuclear reactors and electronic components. Nanoparticles have a diameter of 30 nm and four different values of concentration (from 0 to 6%). The computational domain is made up of the channel and two lateral reservoirs, which simulate the external ambient. The channel is bounded by two vertical parallel plates with a constant aspect ratio. One of the plates is heated with a constant heat flux, while the other is considered adiabatic and is moving in the opposite direction of the buoyancy force. The governing equations are numerically solved with finite volume method by means of the ANSYS-Fluent code. Four different values of wall heat flux (with Ra=7×102 to 106) and six different velocities of the unheated plate (with Re=1 to 102) are considered. Results are presented in terms of velocity, temperature, pressure and stream function. Finally, correlations for the prediction of the average Nusselt number along the heated plate are presented.

Planar network statistics for two-dimensional rupturing foams.

We conduct experiments on a class of two-dimensional semiwet foams generated through compressing a three-dimensional soap foam between two glass plates. To induce a spatially uniform rupturing process on foam boundaries, an additional plate is heated and placed on top of the unheated plates. For 30 separate foam samples, we record network statistics related to cell side numbers and areas as the foam coarsens over a half-minute. We find that the Aboav law and a quadratic Lewis Law, two commonly used relations between network topology and geometry, hold well for preheated foams. To track how well these laws are maintained as the foam ages, we introduce metrics for measuring a foam's disorder over time and build simple autonomous models for these metrics. While the quadratic Lewis Law is found to hold well throughout the rupture process, the Aboav law breaks down rapidly when the Gini coefficient, used for measuring disparity of cell areas, is approximately 0.8.

Heat endurance and impact resistance: Investigating fibrous engineered cementitious composites under impact loads at elevated temperatures

Heat endurance and impact resistance: Investigating fibrous engineered cementitious composites under impact loads at elevated temperatures

Forced Convection Heat Transfer from a Flat Plate at Different Locations between Plates Array.(Dept.M)

A forced convection heat transfer experiment was conducted to study the heat transfer from hot rectangular flat plate ( 155 x 115mm ) to an air stream. The hot plate was situated between unheated plates array at various locations in the flow direction. The study was mainly concerned with the effect of the plate location on the average film coefficient of the transfer. An experimental test rig was designed and constructed for this investigation. The experiments covered the plate locations of x= 12.5, 22.5, 32. 5, 42.5 and 62.5mm, and extended over a range of Revonolds number from about 6,680 to 107, 170. A correlation between the average Nusselt number, the Reynodls number and the hot plate dimensionless location (X) was deduced. The experiments were performed for the case of uniform heat flux. A comparison with the available work in the literature review was also made.

Laminar Forced Convection Over An Inclined Flat Plate With Unheated Starting Length

Two-dimensional laminar forced convection over an inclined flat plate with an unheated starting length was investigated numerically for both constant surface temperature and constant heat flux boundary conditions. The numerical study was implemented using the commercial software ANSYS Fluent 15.0. Air is used as working fluid. The influence of Reynolds number, inclination angle and the length of unheated plate on velocity and temperature distributions, surface temperature, surface heat flux and local Nusselt number was investigated. The results show that Reynolds number, inclination angle and the length of unheated region of plate play important role on heat transfer from the plate. It is seen that Nusselt number increases with increasing Reynolds number and inclination angle of inclined flat plate but decreases with increasing the length of unheated region of plate.

Experimental study on mixed convection in an asymmetrically heated, inclined, narrow, rectangular channel

Abstract Experimental study on mixed convection in the entrance region of a one-side-heated narrow rectangular channel has been carried out. We have performed a series of experiments under natural circulation conditions with Re ranging from 1000 to 3000 and inclination angle ranging from 0° to 30°. Meanwhile, we conducted flow visualization experiments to identify secondary flow driven by temperature difference between the lower, heated and upper, unheated plate of the channel. It is found that thermal instability can be enhanced by increasing the inclination angle of the channel. The secondary flow induces the onset of thermal instability (OTI) while increasing in Re delays the OTI. A sudden increase of hear transfer coefficient and friction factor has been observed after the OTI. The traditional identification criteria for mixed convection are not suitable for the inclined, narrow rectangular channel with a heated lower side. However, transverse Richardson number can identify mixed convection in the channel. The experiment results, together with image data for the flow condition in the inclined narrow rectangular channel, offer valuable data to improve the engineering design of plate-type fuel assembly and similar heat exchangers.

Experimental Investigation and Computational Fluid Dynamics Analysis of Flow and Heat Transfer for Heated Plate

This study investigated the characteristics of convective heat transfer and fluid flow for a heated plate surrounded by unheated plates aligned at angles to the flow direction inside a rectangular duct. The plates were arrayed in two main geometries: inline and staggered. Flow deflections within the array of aligned plates were dominated by boundary-layer separation effects and wake interactions, which in turn influenced the convection heat transfer. The heat transfer coefficient associated with the heated (instrumented) plate was determined by its position and its angle. The numerical model for the heated plate within a bank of plates was validated on experimental data for similar flow geometries and experimental conditions. The heat transfer coefficient decreased in the following rows and at an increased plate angle with respect to the direction of flow. The pressure drop across the array of plates also increased when the plate angles were increased with respect to the direction of flow velocity.

Extension and optimization of a three-layer method for the estimation of thermal conductivity of super-insulating materials

The three-layer method is a characterization method dedicated to the measurement of the thermal conductivity of small samples made out of insulating or super-insulating materials. It is based on the transient recording of the mean temperatures of two brass plates placed, respectively, above and under the sample, one of them being heated. In case of a super insulating material, the rate of heat flow from the heated to the unheated plate through the surrounding air is not negligible with respect to the rate of heat flow through the sample. It is shown that introduction of a simple parallel thermal resistance allows taking this flow of heat into account. An analytical model, based on a new quadrupolar approach, was developed to estimate this resistance considered further as a known parameter in the estimation process. An experimental study was carried out to characterize samples whose thermal conductivities have been measured previously with another method. The deviation of the estimated thermal conductivities from these reference values with respect to the three-layer method results reaches 29% without considering parallel heat flow but is lower than 2% using the new model.

How to Adapt a Lab-Scale Freeze Dryer for Assessing Dehydrating Curves at Different Heating Conditions

A laboratory-scale freeze dryer was adapted to allow control of the heat supply and on-line monitoring of sample weight during drying under vacuum condition. Several tests were carried out to verify the reliability of the developed system. The system was also tested with 10-mm-thick banana slices, obtaining the fruit drying rate, at different sample holder plate temperatures including 20 and 30°C as well as an unheated plate. The system presented in this work represents a low-cost, flexible, and easy-to-assemble piece of equipment that allows the study and optimization of the freeze drying of foods.

Turbulent mixed convection in a uniformly heated vertical channel with an assisting moving surface

Abstract A numerical investigation of mixed convection in air due to the interaction between a buoyancy flow and the flow induced by a moving plate in a vertical channel is carried out. An unheated plate moves at a constant velocity in the direction of the buoyancy force and the principal walls of the channel are heated at uniform heat flux. The effects of the channel spacing, heat flux and moving plate velocity are investigated and results in terms of the channel walls and moving plate temperatures, air velocity profiles inside the channel, wall stress along the channel wall and the moving plate and Nusselt numbers are given. A composite correlation between Nusselt number and Reynolds and Richardson numbers is proposed in the range from natural convection to forced convection up to Re = 1.32 × 10 4 and 9.86 × 10 −6 ≤ Ri ≤ 1.15 × 10 3 , for the channel aspect ratio in the [20.3–81.2] range.

Shape optimization of a flat channel with an array of discrete, flush‐mounted heat sources on one plate being cooled by forced convective water

PurposeThe purpose of this paper is to provide a suitable linkage between a computational fluid dynamics code and a shape optimization code for the analysis of heat/fluid flow in forced convection channels normally used in the cooling of electronic equipment.Design/methodology/approachA parallel‐plate channel with a discrete array of five heat sources embedded in one plate with the other plate insulated constitutes the starting model. Using water as the coolant medium, the objective is to optimize the shape of the channel employing a computerized design loop. The two‐part optimization problem is constrained to allow only the unheated plate to deform, while maintaining the same inlet shape and observing a maximum pressure drop constraint.FindingsFirst, the results for the linearly deformed unheated plate show significant decrease in the plate temperatures of the heated plate, with the maximum plate temperature occurring slightly upstream of the outlet. Second, when the unheated plate is allowed to deform nonlinearly, a parabolic‐like shaped plate is achieved where the maximum plate temperature is further reduced, with a corresponding intensification in the local heat transfer coefficient. The effectiveness of the computerized design loop is demonstrated in complete detail.Originality/valueThis article offers a simple, harmonious technique for optimizing the shape of forced convection channels subjected to pre‐set design constraints.

Experimental Investigation of Opposing Mixed Convection in a Channel with an open Cavity Below

In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant.

Unsteady Natural Convective Boiling in a Narrow Vertical Channel

The purpose of this work is to describe the two-phase flow structure and heat transfer of unsteady natural convective boiling in a narrow vertical channel. The experiments are performed with saturated n-pentane at a pressure of 1 bar. An unheated plate is placed parallel to the heating surface and lateral sides are closed. The distance between the heated surface and the confinement plate is 800 μm. Void fraction measurements are performed using capacitive sensors. The void fraction increases with heat flux and reaches a maximum of 0.80 in the mid-height of the channel when the heat flux is equal to 90% of the critical heat flux. Flow observations using a high-speed video camera show an unsteady thermo-hydraulic behavior. The frequency of the cycles increases with the wall temperature during nucleate and transient boiling. Local velocities of the bubble meniscus developing within the confined space are determined during the boiling cycles. The time-averaged liquid flow rate increases significantly with heat flux and reaches a maximum for heat flux close to the critical heat flux.

Measured turbulent entropy production with large eddy particle image velocimetry

In this paper, statistical post-processing of measured velocity, dissipation rate and turbulence data is performed to establish whole-field distributions of entropy production within a channel. Thermal irreversibilities arising from temperature variations were not included in the study, as the experiments were conducted between unheated plexiglass plates in an essentially isothermal water tunnel. Unlike velocity or temperature, the measurement of entropy cannot be performed directly, so entropy production is measured indirectly through spatial differencing of measured velocities in large eddy PIV. In contrast to single-point methods of anemometry, large eddy PIV enables whole-field, time-varying measurements of the velocity field, which can be post-processed to yield entire spatial variations of the entropy production rate. An uncertainty analysis is performed to estimate measurement uncertainties with the new experimental technique. The uncertainties are decomposed into systematic and random components, including a propagated uncertainty, due to spatial differencing of the velocity field. Close comparisons between measured results of turbulence dissipation and direct numerical simulations provide useful verification of the formulation, before post-processed results of dissipation rates are used to determine entropy production within a channel.

Experimental Investigation of Mixed Convection in a Channel With an Open Cavity

Mixed convection in an open cavity with a heated wall bounded by a horizontally unheated plate is investigated experimentally. The cavity has the heated wall on the inflow side. Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of wall temperature profiles of the heated wall and flow visualization for Re = 100 and 1000, Ri in the range 30–110 (for Re = 1000) and 2800–8700 (for Re = 100), the ratio of the length to the height of cavity (L/D) is in the range 0.5–1.5, and the ratio of the channel height to cavity height (H/D) is in the range of 0.5 and 1.0. The present results show that the maximum dimensional temperature rise values decrease as the Reynolds and the Richardson numbers decrease. The flow visualization points out that for Re = 1000 there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100 the effect of a stronger buoyancy determines a penetration of thermal plume from the heated plate wall into the upper channel. Nusselt numbers increase when L/D increase in the considered range of Richardson numbers.

ISS experiments for the clarification of boiling and two-phase flow in microgravity and for the development of high-performance space cold plates

Inflow boiling, gravity effects on the distribution of both phases are observed in a heated tube and heat transfer coefficients due to two-phase forced convection is deteriorated in microgravity. In narrow channels between heated and unheated plates, the increase in subcooling enlarges a size of flattened bubble and reduces the frequency of detachment under microgravity conditions resulting the emphasis of heat transfer deterioration. To clarify reasons for the unknown behaviors of interfacial distribution and corresponding characteristics in heat transfer not easily be clarified through the experiments on ground, the opportunity on the experiments utilizing long-term microgravity duration realized in ISS is required. The experiments on microgravity boiling and two-phase flow are proposed by the collaboration of researchers in five countries. A common test loop is designed to conduct multiple experiments by the interchangeable structures of test sections; a transparent heated tube for the visualized flow boiling, a stainless tube for the measurement of CHF data, a copper surface for the heat transfer data of nucleate boiling with superimposed liquid flows in a duct, a glass heated plate with multiple array of small temperature sensors and transparent heaters for the clarification of mechanisms in nucleate boiling heat transfer, and one or two models of cold plates for practical applications. A direction of researches in the present discipline is proposed based on the existing experimental results and on the idea developed by the present authors.

Development of a High‐Performance Boiling Heat Exchanger by Improved Liquid Supply to Narrow Channels

A two-phase flow loop is a promising method for application to thermal management systems for large-scale space platforms handling large amounts of energy. Boiling heat transfer reduces the size and weight of cold plates. The transportation of latent heat reduces the mass flow rate of working fluid and pump power. To develop compact heat exchangers for the removal of waste heat from electronic devices with high heat generation density, experiments on a method to increase the critical heat flux for a narrow heated channel between parallel heated and unheated plates were conducted. Fine grooves are machined on the heating surface in a transverse direction to the flow and liquid is supplied underneath flattened bubbles by the capillary pressure difference from auxiliary liquid channels separated by porous metal plates from the main heated channel. The critical heat flux values for the present heated channel structure are more than twice those for a flat surface at gap sizes 2 mm and 0.7 mm. The validity of the present structure with auxiliary liquid channels is confirmed by experiments in which the liquid supply to the grooves is interrupted. The increment in the critical heat flux compared to those for a flat surface takes a maximum value at a certain flow rate of liquid supply to the heated channel. The increment is expected to become larger when the length of the heated channel is increased and/or the gravity level is reduced.

Natural convection in vertical channels with an auxiliary plate

Research on natural convection in open channels is very extensive due to its role in many engineering applications such as thermal control of electronic systems. In this paper, a parametric analysis is carried out in order to add knowledge of heat transfer in air natural convection for a symmetrically heated vertical parallel plate channel with a central auxiliary heated or adiabatic plate. The two‐dimensional steady‐state problem is solved by means of the stream function–vorticity approach and the numerical solution is carried out by means of the control volume method. Results are obtained for both a heated and unheated auxiliary plate, for a Rayleigh number in the range 103–106, for a ratio of the auxiliary plate height to the channel plate height equal to 0, 0.5 and 1 and for a ratio of the channel length to the channel gap in the range 5–15. Correlations for maximum wall temperatures and average channel Nusselt numbers are proposed.

Experimental study of measurement for dissipation rate scaling exponent in heated wall turbulence

Experimental investigations have been devoted to the study of scaling law of coarse-grained dissipation rate structure function for velocity and temperature fluctuation of non-isotropic and inhomogeneous turbulent flows at moderate Reynolds number. Much attention has been paid to the case of turbulent boundary layer, which is typically the nonistropic and inhomogeneous trubulence because of the dynamically important existence of organized coherent structure burst process in the near wall region. Longitudinal velocity and temperature have been measured at different vertical positions in turbulent boundary layer over a heated and unheated flat plate in a wind tunnel using hot wire anemometer. The influence of non-isotropy and inhomogeneity and heating the wall on the scaling law of the dissipation rate structure function is studied because of the existence of organized coherent structure burst process in the near wall region. The scaling law of coarse-grained dissipation rate structure function is found to be independent of the mean velocity shear strain and the heating wall boundary condition. The scaling law of the dissipation rate structure function is verified to be in agreement with the hierarchical structure model that has been verified valid for isotropic and homogeneous turbulence.

BOILING OF MIXTURES IN A NARROW SPACE

Abstract Nucleate pool boiling of binary mixtures composed of water and normal propyl alcohol (NPA) within a narrow space between heated and unheated horizontal-parallel surface has been investigated under atmospheric pressure. The electrically heated lower surface is made of a sheet of stainless steel 304 with the dimension of 145 x 22 x 0.05 mm and the unheated upper plate is a stainlesssieel cylinder with a diameter of 20 mm. The periphery of the confined space is open and the size of the gap can be varied from 0 to 10 mm. Experimental results show that the heat transfer coefficient of nucleate pool boiling of binary mixture in a narrow space is much greater than that of conventional nucleate pool boiling. It is due to the evaporation of a thin liquid film under the deformed bubble. Experimental results also show that mass diffusion effect is important in both nucleate pool boiling in a narrow space and in conventional nucleate pool boiling. Based on the experimental results, the microlayer evaporation...