Dimensionless Average Temperature Research Articles

Constructal optimization of a rectangular non-uniform heat generation area with irregular high thermal conductivity material

Heat conduction models with three non-uniform heat generation conditions in a rectangular heat generation area (RHGA) are set up in this paper. Under given areas of the irregular high thermal conductivity material (HTCM) and RHGA, constructal optimization of the RHGA is conducted by changing the distribution of the HTCM and aspect ratio of the RHGA. The minimum dimensionless average temperature difference (DATD) of the RHGA is gained. It is found that the DATDs of the RHGA with linear heat generation distribution are abated by 82.43% and 82.65% after twice optimizations for boundary and central heat sinks, respectively. For the heat generation in one local area of the RHGA, the two decrements are 36.30% and 52.20%, respectively. For the heat generation in multiple local areas of the RHGA, the two decrements are 35.60% and 71.60%, respectively. The optimal structure of the RHGA with heat generation in multiple local areas and heat sink in the center exhibits the best cooling performance compared with the other models discussed in this paper. Moreover, the best cooling performance of the RHGA is at least augmented by 29.49% than those of the previous studies discussed in this paper.

MHD mixed convection and heatlines approach of nanofluids in rectangular wavy enclosures with multiple solid fins

Two dimensional wavy walls rectangular cavity with inclined magnetohydrodynamic has been examined in mixed convection configurations. Triple fins arranged in the upwards ladder were filled within alumina nanoliquid in the cavity. Vertical sinusoidal walls were heated, and the other side was kept cold while both horizontal walls were kept adiabatic. All walls were motionless except the top cavity that was driven to the right. The diversified range of control parameter in Richardson number, Hartmann number, number of undulations, length of the cavity has been performed in this study. The analysis was simulated using finite element method by employing the governing equation formula, and the results were delineated in the form of streamlines, isotherms, heatlines, and comparisons on several relationships between the local velocity in the y-axis line of 0.6, local and average Nusselt number along the heated surface and dimensionless average temperature. The findings revealed that high concentration nanofluids boost the rate of heat transfer without the need to apply any magnetic field. Results found that the best heat mechanisms are natural convection with significant-high Richardson number as well as constructing two waves on the vertical walls in the cavity.

Numerical study of melting and heat transfer of PCM in a rectangular cavity with bilateral flow boundary conditions

The melting of phase change material (PCM) in a rectangular cavity with heat transfer fluids (HTFs) flowing at both sides from bottom up (bilateral flow boundary conditions) are numerically studied. A parametric study is performed by changing the Reynolds number (Re) of HTF in a range of 1 × 104, 3 × 104, 5 × 104, 7 × 104 and the Rayleigh number (Ra) in a range of 3 × 106, 4 × 106, 5 × 106, 6 × 106. The results show that the formation of natural convection is due to the impacts of non-uniform heat transfer of HTF and cooling between solid-liquid interface and liquid PCM. The asymmetric flow boundaries lead to various skew forms of the PCM melting process and effect the heat transfer and thermal energy storage. The melting process can be divided into three melting stages, the first two stages are mainly ruled by Re and the last stage by Ra. A set of introduced dimensionless parameters are analyzed to elaborate the heat transfer mechanism and the general rule of melting characteristics. The various dimensionless correlations are applied to describe the evolution of melting volume fraction, dimensionless average temperature and thermal energy storage fraction during the melting process.

Analytical solution of coupled heat and mass transfer equations during convective drying of biomass: experimental validation

In this work, analytical solutions of the thin-layer and the Luikov models were proposed. For the thin-layer model, the moisture content and temperature distribution inside a biomass, assimilated to a parallelepiped material, are uniform whereas the Luikov model considers the sample as a porous medium. Analytical solutions of the Luikov model equations were obtained using Hermite’s zero-order approximation. These analytical solutions and those of the thin-layer model equations were applied to forced convection drying of two types of biomass: raw olive pomace (ROP) and deoiled olive pomace (DOP). It has been shown that the Luikov model is in good agreement with the experimental results except for the case where the sample has a thickness of 0.5 cm. In addition, thermal properties of ROP and DOP have been determined experimentally. On the other hand, a brief parametric study has been conducted and simultaneous effects of the dimensionless numbers: Luikov (Lu < <1), Kossovitch (Ko) and Posnov (Pn), on the drying process are highlighted. Thus, the increase of Luikov number accelerates the drying process and for specific couples (Lu and Ko) or (Lu and Pn), a remarkable change of dimensionless average water content with respect to dimensionless average temperature is observed, at the beginning of the drying process.

Numerical study of steady natural convection in a liquefied natural gas cylindrical storage tank equipped with baffles

In this paper, a study of cooling system for a liquefied natural gas storage tank is conducted. Our objective is to remedy the heat ingress to the liquefied natural gas from the environment using baffles toward limiting temperature elevation in the tank, and then the Boil-off Gas (BOG) formation. A specific code based on the finite volume method is developed to supply a fine knowledge of the hydrodynamic and thermal liquefied natural gas characteristics in the cylindrical tank heated from bottom and lateral surfaces. The effect of the number, position and dimension of baffles, on the flow structure and thermal behavior, has been analyzed. According to our simulation results, the baffles should be placed at the top of tank nearby the lateral wall as the liquefied natural gas dimensionless average temperature can be reduced by 36%. The installation of four rectangular baffles, equally spaced around the perimeter of the tank, gives better homogenization of the temperature field and decreases the average temperature by about 44% in order to limit BOG formation. Finally, two correlations of the Nusselt number are established for the flat rectangular baffle plates and the lateral surface of the cylindrical liquefied natural gas storage tank as a function of the Rayleigh number, as well as the baffle number. Scaling of these correlations with the Rayleigh number gives exponents of 0.25 and 0.18 for lateral surface and baffle, respectively, which are in good agreement with literature.

THE EFFECT OF WHOLE BODY GAS CRYOTHERAPY ON THE HUMAN THERMAL STATE

The therapeutic effect of the whole body gas cryotherapy (WBGC) procedure is achieved when the average temperature of the human’s (patient’s) skin surface during the procedure is in a certain temperature range, and the rate of reduction of this temperature exceeds a certain threshold value. The paper presents the results of a theoretical analysis of the patient’s thermal state at the stage of the therapeutic effect of WBGC. The analysis was based on the results of the numerical simulation of temperature distribution in different parts of patients (mеn) of different body constitution (height, cm × mass, kg) 160 × 60, 175 × 75, 190 × 90. Particular attention was paid to the relationship between the amount of heat flow removed from the skin surface and the rate of cooling of the skin of a patient. The results of numerical modeling are presented graphically and are summarized as the ratio between the dimensionless average temperature of the skin at the treatment stage of the WBGC procedure and the time of the patient’s stay in the procedural chamber, referred to the time of the beginning of this stage. With regard to WBGC within the temperatures range of –160 to –110 °C the following regularities were found: 1) approximate constancy (descripancy less than 1 %) at the stage of therapeutic modality of the coefficient of heat transfer from the skin surface to the gas medium in the treatment chamber; 2) the exponential dependence of the value of cutaneous blood flow rate (blood perfusion) on the mean temperature of cooled cutaneous coverings; 3) linear dependence of the logarithm of the ratio of the rate of skin temperature reduction to the value of heat flux density removed by convection and radiation on the logarithm time.

Comparison and Application of Two Mathematical Models in Predicting and Determining the State of Deposition and Slagging of Coal Burning Boiler

Building a model to predict the state of slag on coal-fired boilers is a good way to optimize the coal combustion and reduce the risk of boiler slag. This paper built new models based on vague sets to predict the state of slag on coal-fired boilers, in which there were six input vectors, which were softening temperature, SiO2-Al2O3 ratio, alkali-acid ratio, percentage of silicon content, the dimensionless average temperature furnace and the dimensionless inscribed circle diameter furnace, and one output vectors, which was slagging degree. Two methods, which were based on the sense of distance and symmetric fuzzy cross entropy, were proposed to calculate the similarity between vague sets. 10 coal burning boilers were selected as known samples and the feasibility of the new methods was proved by the result of predicting the state of slag on the four coal burning boilers from Jilin heat and power plant, Xinli power plant, Jinzhou power plant and Qinhuangdao power plant. Through predicting and determining, it proves that the two pattern recognition models are high in prediction accuracy. Compared with the normal method, it is easier for operators to predict, determine the slagging state and reduce disturbance as far as possible. Besides, a prediction system has been developed by object-oriented high-level language accordingly.

Mathematical Modeling of Heat and Mass Transfer in Gas Absorption on a Drop of a Volatile Absorbent

A model of simultaneous heat and mass transfer in absorption of component A from a gas mixture in a drop of a volatile absorbent at commensurable phase resistances is constructed based on the known model of heat and mass transfer inside a drop. Heat and mass transfers in the gas phase are described in terms of the heat and mass transfer coefficients. The relation between the dimensionless mole fraction of component A and temperature averaged over the drop volume and the mole fraction and temperature at the drop surface is found using Duhamel’s principle and the balance of the heat and mass flows at the interface. The solutions of the resulting integro-differential equations are presented as a series of exponential functions with constant coefficients, as is the solution of the internal problem. For the process of absorption of ammonia in a water drop, the dimensionless average temperature in the drop as a function of time are given at different concentrations of vapor in the bulk of the dispersion medium.

Forced Convection Heat Transfer to Power-Law Non-Newtonian Fluids Inside Triangular Ducts

A hybrid analytical-numerical approach based on the Generalized Integral Transform Technique is employed to simulate the laminar forced convection (hydrodynamically fully developed and thermally developing laminar flow) of power-law non-Newtonian fluids inside ducts with arbitrary shaped cross-sections. The analysis is illustrated through consideration of both right angularly and isosceles triangular ducts subjected to constant wall temperature as thermal boundary condition. Reference results for quantities of practical interest such as dimensionless average temperature and Nusselt numbers within the thermal entry region were produced for different values of power-law index and apex angles. Finally, critical comparisons are performed with results available in the literature for direct numerical and approximate approaches.

Laminar thermally developing flow inside right-angularly triangular ducts

An analytical approach based on the generalized integral transform technique is presented, for the solution of laminar forced convection within the thermal entry region of ducts with arbitrarily shaped cross-sections. The analysis is illustrated through consideration of a right triangular duct subjected to constant wall temperature boundary condition. Critical comparisons are made with results available in the literature, from direct numerical approaches. Numerical results for dimensionless average temperature and Nusselt numbers are presented for different apex angles.

Transient heat and mass transfer in film absorption of finite depth with nonhomogeneous boundary conditions

A theoretical analysis of transient combined heat and mass transfer processes that occur in liquid desiccant film absorption is presented. Analytical expressions are derived for the temperature and concentration in a stagnant film of finite depth. Comparison with experimental data allows the determination of the effective Lewis number. Using the analytical expressions, the transient variation of the heat and mass rates and the transient dependence of the dimensionless average temperature with Lewis number and the dimensionless enthalpy of absorption are presented.