Computational fluid dynamics and experimental study of turbulent natural convection coupled with surface thermal radiation in a cubic open cavity

https://doi.org/10.1016/j.ijmecsci.2021.106360Get rights and content

Highlights

  • 1

    The research contains the following novelty aspects:

  • 2

    An experimental setup was built to obtain temperature profiles and heat transfer coefficients, for the turbulent natural convection coupled with thermal radiation in an open cavity.

  • 3

    The physical system was modeled with five different turbulence models and a comparison was made between experimental data and numerical results to validate the mathematical model.

  • 4

    A detailed 3D numerical analysis was conducted of airflow and heat transfer in different open cavities configurations.

  • 5

    The effect of the wall emissivity is shown and quantified experimentally and numerically.

Abstract

Turbulent heat transfer in open cavities is relevant in several thermal engineering applications. However, the influence of thermal radiation has not been thoroughly studied yet. In this work, experimental and numerical results were obtained to analyze turbulent natural convection coupled with surface thermal radiation in a cubic open cavity. Two emissivities were considered (0.98 and 0.03). Experimental temperature profiles were obtained at six different depths and heights consisting of 14 thermocouples each. Five turbulence models were evaluated against experimental data. The radiative heat transfer model was solved with the discrete ordinate method. The validated model was used to analyze temperature fields and flow patterns in different open cavities configurations, which are presented and discussed. The effect of thermal radiation on experimental heat transfer coefficients was quantified. It was found that experimental heat transfer coefficients increase between 8.5% and 12.52% when the emissivity of the walls changes from 0.03 to 0.98. The temperature distribution in the vertical adiabatic walls changes significantly when emissivity increases from ε=0.03 to ε=0.98 and thus have a relevant influence on the flow patterns in the open cavity.

Introduction

Turbulent heat transfer in open cavities is relevant in several thermal engineering applications. In some concentrating solar power technologies (CSP), like solar power tower or dish-Stirling, thermal receivers may have an open cavity configuration to reduce thermal losses. Besides working with high temperatures, surfaces of solar receivers are covered with selective black paints to absorb the maximum quantity of incoming solar radiation. Therefore, to compute thermal losses, radiative exchange, must be included.

In the literature, several studies have been reported to describe the heat transfer in open cavities by natural convection and thermal radiation. These studies can be categorized as (a) Numerical studies of laminar natural convection with thermal radiation and (b) Experimental studies with thermal radiation. The previous investigations are briefly presented next.

Lage et al. [1] reported results for laminar natural convection and surface thermal radiation in a two-dimensional open-top cavity. Mathematical models for natural convection and surface thermal radiation were solved separately by assuming a temperature distribution on the vertical adiabatic walls. They included the individual effects of the dimensions of the cavity, the temperature of the heated wall, and the emissivities of the two side walls. Balaji and Venkateshan [2] analyzed the heat transfer by laminar natural convection coupled with surface thermal radiation in a two-dimensional open-top cavity. The left wall was considered isothermal, and the right and bottom walls were adiabatic and their temperature distributions were determined by an energy balance between convection and radiation in each surface element of the walls. It was found that surface thermal radiation changes the flow pattern and the overall heat transfer coefficient substantially. Balaji and Venkateshan [3] reported a study of combined conduction, laminar natural convection, and surface thermal radiation in a two-dimensional open-top cavity, reporting that surface thermal radiation increases overall heat transfer between 50 and 80 % depending on the radiative parameters.

Singh and Venkateshan [4] presented results of steady laminar natural convection and surface thermal radiation in a two-dimensional side open cavity. It was observed that surface radiation induces the formation of thermal boundary layers along adiabatic walls. Dehghan and Behnia [5] made a study of combined laminar natural convection, conduction, and surface thermal radiation in a two-dimensional open-top cavity with a discrete heat source. The surface emissivity was varied. It is noted that the inclusion of radiation has a significant effect on the flow, resulting in the formation of a recirculation zone within the cavity. Hinojosa et al. [6] reported Nusselt numbers for a tilted open square cavity, considering laminar natural convection and surface thermal radiation. The inclination angles for the cavity were in the range of 0° to 180°. It is observed that the convective Nusselt number changes substantially with the inclination angle of the cavity, whereas the radiative Nusselt number stays practically constant.

Hinojosa et al. [7] developed a transient and steady-state study of laminar natural convection with surface thermal radiation in an open square cavity. The results showed that the radiative exchange increases considerably the total average Nusselt number, from around 94% to 125%. Wang et al. [8] made a numerical investigation in a two-dimensional open cavity with laminar natural convection, conduction, and surface radiation. It was reported that radiation and solid conduction increase the average total Nusselt number, and it has a linear increase with emissivity for emissivity larger than 0.2. Nouanegue et al. [9] investigated numerically heat transfer by laminar natural convection, conduction, and thermal radiation in a two-dimensional open cavity with constant heat flux in the wall facing the aperture. They report a considerable effect of the thermal radiation on the flow and temperature fields and that the heat flux by radiation increases with the emissivity. Hinojosa-Palafox [10] performed a two-dimensional numerical study in a tilted open shallow cavity. It was reported the relevance of thermal radiation exchange between walls for an inclination angle of 135°, as well as oscillations on the convective Nusselt number for an inclination angle of 45°.

Montiel-Gonzalez et al. [11] analyzed the validity of the Boussinesq approach to predict the heat transfer by laminar natural convection coupled with surface thermal radiation in a square open cavity. For total Nusselt numbers, the results with the Boussinesq approach and variable properties indicate deviations within 0.22 % and 5 %. Hinojosa et al. [12] made a numerical study on entropy generation in a square open cavity with laminar natural convection and surface thermal radiation. The results of this investigation indicate that surface thermal radiation increases the overall entropy generation rate between 33.52% and 560.87%.

Ramesh and Merzkirch [13] developed an experimental setup to study laminar natural convection with thermal radiation in a side open cavity with a top opening. They studied different aspect ratios, surface emissivities and side vents with a vertical heated wall and the remaining walls are adiabatic, estimating radiative heat transfer rates. Wu et al. [14] reported heat losses in an open cylindrical cavity for different boundary conditions, tilt angles, and heat flux. It was found that the laminar natural convection heat losses depend on the tilt angle, whereas the inclination angle causes a little effect on the radiative heat losses.

As seen in the literature analysis, there are no combined experimental-numerical three-dimensional studies on the effect of surface thermal radiation on turbulent natural convection in open cavities. Considering the above, this work reports a detailed experimental and numerical analysis of thermal radiation coupled with turbulent natural convection in a cubic open cavity. The effect of thermal radiation on experimental heat transfer coefficients was quantified by considering two different wall emissivities (0.98 and 0.03). In the numerical study were tested five turbulence models of the Reynolds Averaged Navier-Stokes (RANS) family in combination with the discrete ordinate method (DOM), to establish which has a better agreement with experimental heat transfer coefficients and temperature profiles. The validated model was used to obtain temperature fields and flow patterns in different open cavities configurations, which are presented and discussed.

Section snippets

Physical model

Heat transfer by natural convection and thermal radiation in an open cavity with an edge (L) of 1 m (Fig. 1) is analyzed. It consists of one vertical wall with a constant heat flux (q¨H), which is facing the aperture. The remaining walls are adiabatic. This configuration is like a thermal receiver that receives solar radiation in one heated wall to transfer thermal energy to the working fluid, whereas the remaining walls are thermally insulated to avoid thermal losses. The airflow inside the

Experimental procedure

The hot wall of the cavity consists of a flexible electrical heater in contact with a thin sheet of polished aluminum (ε=0.03). It is supported in a box made of 0.0254 m plywood with 0.20 m of thermal insulation (0.1 m of mineral wool and 0.1 m of polystyrene). The electrical heater is connected to an Agilent DC power supply model E3632A, which allows to regulate the electrical tension and to produce the desired thermal power. The adiabatic walls of the cavity were built with medium density

Numerical methodology

The mathematical model was solved using ANSYS Fluent 15. The SIMPLEC algorithm [22] was used to coupling the momentum and continuity equations, as well as the MUSCL third order scheme to discretize the convective terms [23]. The discrete ordinate method was used to solve the radiative heat transfer equation. The convergence was achieved when the weighted residue of the governing equations was less than 10−4, nevertheless, for the energy equation, it was less than 10−6. To determinate the

Results and discussion

The experimental and numerical results are shown in the followings sections: (1) Comparison between experimental temperature profiles for ε=0.98 and ε=0.03, (2) Comparison between experimental and numerical temperature profiles for ε=0.98, (3) Comparison between numerical temperature contours and velocity vectors for ε=0.03 and ε=0.98, (4) Comparison between heat transfer coefficients and average Nusselt numbers for ε=0.03 and ε=0.98.

Conclusions

This work has analyzed the effect of thermal radiation on turbulent natural convection in open cavities. From the results, we establish the following conclusions:

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    The Re-Normalization group k-ε model (rngkε) has better agreement with the experimental temperature profiles and heat transfer coefficients.

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    Experimental heat transfer coefficient increases between 8.5% (Ra = 1.2  ×  1012) and 17.64% (Ra = 2  ×  1011) when the emissivity of the walls changes from 0.03 to 0.98.

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    The temperature

CRediT authorship contribution statement

J.M.A. Navarro: Software, Validation, Investigation, Writing – original draft, Writing – review & editing. J.F. Hinojosa: Conceptualization, Methodology, Writing – review & editing, Supervision, Funding acquisition. A. Piña-Ortiz: Writing – review & editing, Visualization, Formal analysis.

Declaration of Competing Interest

The author(s) declared no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

Acknowledgment

The first author wishes to acknowledge the support of the National Science and Technology Council of the Mexican Republic, given through its graduate grants program.

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