Experimental Study Of Mixed Convection Research Articles

An experimental study on the heat transfer characteristics over a radiant cooled wall exposed to mixed and forced convection driven by displacement ventilation

An increasing demand on energy efficiency and indoor air quality exists, therefore displacement ventilation and radiant surface cooling applications are taking more attention as an investigation subject. This article presents an experimental study of mixed convection in a test chamber equipped with both radiant wall cooling and air-conditioning systems. The room is conditioned by a hydronic circuit with embedded pipes in the wall and an air jet supplied by a newly designed baseboard longitudinal slot that is apart from relevant literature. The tested radiant wall temperature is kept around 14 °C. Meantime; a parallel upward directional air jet is applied over the cooled wall or the opposite wall. The air jet velocity and its temperature lie within the intervals of 0.25–10 m/s and 18–28 °C, respectively. The natural, mixed, and forced convection heat transfer characteristics over the wall have been investigated considering the effects of radiant cooling, airflow rates, and ventilation location. The measured data are used for the derivation of mixed and forced average convective heat transfer coefficients correlations along a cooled wall. Consequently, the convection and total heat transfer coefficients over the radiant cooled wall are calculated in the ranges of 2–7.3 W/m2K, and 9.1–13.9 W/m2K, respectively.

Poiseuille-Rayleigh-Bénard mixed convection flow in a channel: Heat transfer and fluid flow patterns

In the present work, a numerical and experimental study of mixed convection of water flow in a horizontal channel subjected to a constant heat flux is presented. An experimental setup was constructed to delineate the fluid flow patterns inside the channel, while a numerical simulation was performed to study the heat transfer characteristics of the flow. This study is performed for Rayleigh and Reynolds numbers in the range of 104 ≤ Ra≤ 106 and 25≤ Re≤ 100, respectively. The present predicted results are in good agreement with the experimental ones. The longitudinal evolution rolls revealed the existence of four zones corresponding to the entry zone, establishment zone, destabilization zone, and turbulent zone. Moreover, numerical simulations showed that the Nusselt number for the mixed convection gives higher heat transfer coefficient compared to forced convection. In addition, new correlations for the Nusselt number have been developed for the first time for the establishment length, onset and established zones as a function of Rayleigh and Reynolds numbers.

Comparative analysis of experimental investigations and numericalsimulation of single-row finned tube bundle at mixed convection

Mixed convective heat transfer is very important for a wide class of engineering tasks. However, the experimental study of mixed convection requires significant implementation costs, high-power equipment, as well as large time costs, so it is proposed to expand the scope of experimental studies using numerical simulation. Numerical simulation of the single-row bundle consisting of bimetallic finned tubes at mixed air convection conditions was performed and experimental data were compared. The formulation of the third-dimensional problem for numerical simulation was realized. The conjugated problem for heat exchange modeling from the tube fins to air was solved. In numerical simulation of air momentum it was taken into account that the Reynolds number based on tube diameter and velocity in the space between fins was varied from 100 to 720. Menter’s k–ω shear stress transport model in standard formulation was used to close the Reynolds equations. Flow visualization on the tube surface revealed the transient nature of the air flow. The temperature distribution visualization in the bundle and the exhaust mine made it possible to see the nature of cooling the finned bundle at mixed convection. Results of numerical simulation and experimental investigations are in good agreement and can be used for expansion of the scope of experiments. The experimental data and the numerical simulation results for the single-row bundle consisting of bimetallic finned tubes at mixed air convection are compared in this paper. Flow near tube surfaces was visualized, and the temperature and velocity distributions in a bundle and in the exhaust mine were obtained.

An experimental study of mixed convection over various thermal activation lengths of vertical TABS

This paper presents experimental study of supported mixed convection of two-dimensional wall jet over various thermal activation lengths of vertical TABS. It was found out that supported mixed convection upward the jet over vertical TABS is not influenced by thermally inactivated vertical TABS downwards of the wall jet. Practical consequences of this founding were after that tested using TRNSYS simulation tool in energy simulation study for contemporary modern office room having vertical TABS. Results show that office room can be sufficiently cooled by mixed convection even for TABS designs where only upper half of room wall is thermally activated by environmental high temperature cooling sources.

Experimental study and mathematical simulation of the mixed convection in a rectangular area with a local heat source and the heat sink at the external boundaries

The results of the experimental determination of temperatures and numerical simulation of temperature fields in a mixed convection in a rectangular area with a heat-conducting walls at a local energy source on the bottom are presented. For the experimental determination of temperature fields in a mixed convection method of thermocouple measurements was used. Studies were conducted at Reynolds numbers 10 ≤Re ≤ 1500 and Rayleigh 103 ≤Ra ≤ 105. For the verification of obtained experimental data mathematical simulation of mixed convection in the region with the use of a software package Comsol Multiphysics is carried out. A good agreement between the results of experimental determination of the local characteristics of mixed convection in a rectangular volume with a local source of energy and heat-conducting walls with theoretical consequences obtained in the simulation of convective flows using a mathematical package is established. It makes it possible to make a conclusion about the possibility of application for the analysis of flow conditions of viscous heat-conducting gas of experimental procedure based on methods of thermocouple measurements. At the same time the developed approach to the experimental study of mixed convection is applicable in a wide enough range of the main flow parameters and the experimental conditions.

Experimental study of mixed convection with water–Al 2O 3 nanofluid in inclined tube with uniform wall heat flux

An experimental investigation was carried out to study mixed convection of Al 2O 3–water nanofluid inside an inclined copper tube submitted to a uniform wall heat flux at its outer surface. The effects of nanoparticles concentration and power supply on the development of the thermal field are studied and discussed under laminar flow condition. Results show that the experimental heat transfer coefficient decreases slightly with an increase of particle volume concentration from 0 to 4%. Two new correlations are proposed to calculate the Nusselt number in the fully developed region for horizontal and vertical tubes, for Rayleigh number from 5 × 10 5 to 9.6 × 10 5, Reynolds number from 350 to 900 and particle volume concentrations up to 4%.

DUCT FLOW PASSING THROUGH A HORIZONTAL CHANNEL WITH TANDEM CYLINDER SOURCES

The purpose of present study is to investigate laminar fully developed flow in a horizontal rectangular channel with the tandem cylinder sources under different Re and source positions. The boundaries in this experiment include four cases: a. the former cylinder is cathode and the latter cylinder is anode; b. the former cylinder is anode and the latter is cathode; c. the upper plate is cathode and two cylinders are anode; and d. the two cylinders are cathode and upper plate is anode. The influence of the mass transfer rate and the boundary types between the sources have also been investigated. An experimental study of mixed convection with CuSO4 plus; H2SO4 + H2O solution in a horizontal channel was performed. The shadowgraph technique was used to visualize the flow and to determine the nature and effect of solutal driven secondary flows in a horizontal channel with the tandem cylinder sources inside. The ranges of the parameters studied in the present work are Pr = 5–7, Ar = 1, Sc = 1700–2400, Re = 50–200, Grm = 1.75 × 105 (Red = 7.25–29, Gr*m = 6833.6), d/H = 0.145.

Experimental study of mixed convection and pressure drop in helically dimpled tubes for laminar and transition flow

This paper presents the experimental results carried out in dimpled tubes for laminar and transition flows and completes a previous work of the authors focused on the turbulent region. It was observed that laminar flow heat transfer through horizontal dimpled tubes is produced in mixed convection, where Nusselt number depends on both the natural convection and the entry region. Employing water and ethylene glycol as test fluids, the following flow range was covered: x ∗=10 −4–10 −2 and Ra=10 6–10 8. The experimental results of isothermal pressure drop for laminar flow showed dimpled tube friction factors between 10% and 30% higher than the smooth tube ones. Moreover, it was perceived that roughness accelerates transition to critical Reynolds numbers down to 1400. Correlations for the laminar friction factor f= f( Re, h/ d) and for the critical Reynolds Re crit= Re crit( h/ d) are proposed. The hydraulic behaviour of dimpled tubes was found to depend mainly on dimple height. In mixed convection, high temperature differences in the cross section were measured and therefore heat transfer was evaluated by a circumferentially averaged Nusselt number. Experimenal correlations for the local and the fully developed Nusselt numbers Nu x= Nu x(x ∗,Ra) and Nu ∞= Nu ∞(Ra) are given. Results showed that at low Rayleigh numbers, heat transfer is similar to the smooth tube one whereas at high Rayleigh, enhancement produced by dimpled tubes can be up to 30%.

An experimental study of mixed convection around a rotating isothermal cylinder

The mixed convective flows generated by a heated rotating horizontal cylinder have been investigated experimentally. The presence of rotation represents an important complication and extension of previous studies on convective flow and heat transfer around heated stationary horizontal cylinders in an otherwise quiescent medium. Significant qualitative, as well as quantitative, differences in the heat transfer characteristics and flow patterns generated by the rotating cylinders are observed as compared to those for stationary cylinders. When the Reynolds number reaches a value corresponding to the rotational parameter σ ≡ Gr/Re 2 being just below unity, the thermal plume becomes unstable and eventually breaks down. This event corresponds to the onset of a secondary mean flow which appears to be periodic in the axial direction. Mean Nusselt number measurements are presented, which show a dramatic increase in heat transfer at the onset of the secondary flow. Also, the structure of the secondary flow and the behavior of the plume are elucidated through Schlieren photography.

Analytical and experimental study of mixed convection in tube bundles with a longitudinal flow

A solution is given to the heat problem of mixed convection in vertical tube bundles with a relative spacing s/d=1.2 and 1.4. The results have been checked out experimentally with a forced flow of liquid sodium.

Experimental study of combined free and forced laminar convection in an asymmetrically heated two-dimensional aiding flow

An experimental study of mixed convection in an asymmetrically heated two-dimensional flow of water has been made. Experiments in fully developed turbulent flow (Re maximum=9000) were made initially to establish the satisfactory operation of the equipment. In the mixed convection laminar flow tests to determine local Nusselt numbers, the Reynolds number range was from about 100 to 1000 with the Rayleigh number based on heat flux varying from about 10 to 300. The data, which are presented graphically, have been compared with the results of the theoretical analyses of other workers. The effect of buoyancy in the immediate entry of the flow [(x/de)/Re Pr <.05] was found to be negligible, but at greater distances there were significant increases in the heat transfer coefficient with the larger values of Rayleigh number.