Numerical investigations of solar cell temperature for photovoltaic concentrator system with and without passive cooling arrangements

The numerical study of solar cell temperature for concentrating PV with concentration ratio of 10× is presented in this paper. A two dimensional thermal model has been developed to predict the temperature for PV concentrator system (solar cell and lens) with and without passive cooling arrangements. Based on a thermal model, the result shows that maximum of four numbers of uniform fins of 5 mm height and 1 mm thickness can be effectively used to reduce the solar cell temperature. In addition to that, the effects of ambient temperature and solar radiation intensity on the solar cell temperature have also been investigated for the system with and without cooling fins. Based on the influencing parameters of ambient temperature and solar radiation, two separate solar cell temperature correlations has been proposed for systems with and without cooling fins to predict the cell temperature for the range of given parameters. In our previous studies, the present 2-D model was extensively validated with a comprehensive unified model [8], [9] and [10].     

Dendritic heat convection on a disc

In this paper we develop the optimal tree-shaped flow paths for cooling a disc-shaped body by convection. Heat is generated uniformly over the disc area. The coolant enters through the center of the disc, and exits through ports positioned equidistantly along the perimeter. The unknown is the flow architecture. The constraints are the disc size and the total volume occupied by the ducts. It is assumed that the ducts are narrow enough so that the flow is hydrodynamically and thermally fully developed. The ultimate goal is to determine flow architectures that reach simultaneously two objectives: (i) minimal global fluid flow resistance (or pumping power), and (ii) minimal global thermal resistance. When the architecture is optimized for (i), the result is a dendritic structure in which every geometric feature is uniquely determined. The corresponding thermal resistance decreases as the total mass flow rate and the pumping power increase. When the objective is (ii), the optimal architecture has radial ducts, not dendrites. The corresponding fluid-flow resistance increases as the flow rate increases and the global thermal resistance decreases. Put together, these geometric results show that methods (i) and (ii) lead to nearly the same combined performance (thermal and fluid). Examined more closely, the dendrites produced by method (i) perform progressively better as the length scales become smaller. Optimized increasing complexity is the route to high thermal and fluid-flow performance in the limit of decreasing scales.     

NUMERICAL SIMULATION OF TURBULENT CONVECTIVE HEAT TRANSFER IN SQUARE RIBBED DUCTS

This article reports on an investigation on numerical prediction of thermal characteristics of a certain type of duct. The ducts considered have rib turbulators to enhance the heat transfer rate. The calculation method consists of a low Re number turbulence model and two methods for determining the turbulent Reynolds stresses, namely, a simple eddy viscosity model (EVM) [1] and an explicit algebraic stress model (EASM) [2]. The model development is carried out to make the original EASM consistent with the low Re number k- epsilon turbulence model applied. A certain method is developed to deal with the decoupling of the velocity and Reynolds stress fields inthe collocated grid arrangement that is chosen in this study. The SIMPLEC algorithm handles the pressure-velocity coupling. The computations are performed with the assumption of fully developed periodic conditions. These models are used to predict the convective turbulent forced convection in different test cases and the results are compared with experiments. A ribbed duct with two ribs on opposite walls is chosen and the obtained results including the mean thermal characteristics of the considered duct are compared with an experimental correlation. Two further duct configurations, identical to an experimental setup, are then computed. These experimental cases are chosen because detailed thermal-hydraulic information is available and then local comparisons between the two prediction models and experimental results are possible. The calculated mean and local thermal-hydraulic values are compared with corresponding experimental data and the prediction capabilities of the two turbulence models (EVM and EASM) are discussed. Theresults show that the EASM has some superiority over the EVM in the prediction of the velocity field structure, but the mean thermal predictions are not very different. There are also some important features of the flow field, whichare not revealed by theEVM calculations. However, the required CPU times are considerably higher for the EASM case.     

Intense heat simulation studies on window of high density liquid metal spallation target module for accelerator driven systems

The design of the spallation target system for accelerator driven systems requires a detailed understanding of the thermal-hydraulic issues involved as intense heat is deposited both in the window as well as in the target. Removal of heat from the window is a big thermal-hydraulic challenge. A lead–bismuth eutectic (LBE) experimental flow loop is currently being designed (1:1 size of an actual target both in terms of geometry and flow rate) to study thermal-hydraulics of the target system. It is proposed to simulate the proton beam heating of the window with a plasma heat source. In this paper, flow simulation studies have been performed for different target geometries and flow configurations with different window materials, which are proposed for the above said experimental loop. Optimal values of window geometry and flow configurations including the thermal loads the window experiences have been arrived at along with the required parameters for the plasma torch. The plasma torch has been tested and found to be suitable for simulation of proton beam heating in LBE loop to be set up.     

A study of the air-side heat transfer and pressure drop characteristics of tube-fin ‘no-frost’ evaporators

A study is presented on the influence of the air flow rate and surface geometry on the thermal-hydraulic performance of commercial tube-fin ‘no-frost’ evaporators. A specially constructed wind-tunnel calorimeter was used in the experiments from which data on the overall thermal conductance, pressure drop, Colburn j-factor and Darcy friction factor, f, were extracted. Eight different evaporator samples with distinct geometric characteristics, such as number of tube rows, number of fins and fin pitch were tested. Semi-empirical correlations for j and f are proposed in terms of the air-side Reynolds number and the finning factor. A discussion is presented on the performance of the evaporators with respect to specific criteria such as the pumping power as a function of heat transfer capacity and the volume of material in each evaporator.     

Optimal active cooling performance of metallic sandwich panels with prismatic cores

All-metallic sandwich panels with prismatic cores are being currently investigated for combined structural and active cooling performance. We present a new approach to active cooling performance, and use it to optimize the panel geometry for four different systems: aluminum-air, aluminum-water, aluminum-gasoline and titanium-gasoline. The results show that some geometric parameters can be fixed without much detriment in thermal performance. Moreover, while optimal core densities are typically 25–50%, near-optimal results can be obtained with densities as low as 10%. These findings provide considerable geometric flexibility when attempting combined thermal and structural optimization.     

Constructal design of T–Y assembly of fins for an optimized heat removal

Constructal design has been applied to a large variety of problems in nature and engineering to optimize the architecture of animate and inanimate flow systems. This numerical work uses this method to seek for the best geometry of a T–Y assembly of fins, i.e., an assembly where there is a cavity between the two branches of the assembly of fins. The global thermal resistance of the assembly is minimized by geometric optimization subject to the following constraints: the total volume, the volume of fin-material, and the volume of the cavity. Parametric study was performed to show the behavior of the twice minimized global thermal resistance. The results show that smaller cavity volume and larger fins volume improve the performance of the assembly of fins. The twice minimized global thermal resistance of the assembly and its corresponding optimal configurations calculated for the studied parameters were correlated by power laws.     

Numerical investigation of H2 absorption in an adiabatic high-temperature metal hydride reactor based on thermochemical heat storage: MgH2 and Mg(OH)2 as reference materials

A two-dimensional mathematical model to predict the thermal performance of an adiabatic hydrogen storage system based on the combination of magnesium hydride and magnesium hydroxide materials has been developed. A simple geometry consisting of two coaxial cylinders filled with the hydrogen and thermochemical heat storage materials was considered. The main objective was to gain a better knowledge on the thermal interaction between the two storage media, and to determine the dependence of the hydrogen absorption time on the geometric characteristics of the reactor as well as the operation conditions and the thermophysical properties of the selected materials. The dimensions of the two compartments where the two materials are filled were chosen based on the results of a preliminary analytical study in order to compare the absorption times obtained analytically and numerically. The numerical results have shown that the hydrogen absorption process can be completed in a shorter interval of time than analytically as a result of the larger temperature gradient between the magnesium hydride and magnesium hydroxide beds. This was mainly due to variation of temperature in the thermochemical heat storage material during the more realistic dehydration reaction in the numerical solution. Larger temperature gradients, thus a faster hydrogen absorption process can also be achieved by increasing the hydrogen absorption pressure. Moreover, it was found that the increase of the thermal conductivity of the magnesium hydroxide material is crucial for a further improvement of the performance of the MgH₂–Mg(OH)₂ combination reactor.     

Effect of cooling channel geometry on re-cooled cycle performance for hydrogen fueled scramjet

Developing fuel with higher heat sink is widely carried out to meet the cooling requirement for an airbreathing hypersonic vehicle. However, a Re-Cooled Cycle has been newly proposed for a regeneratively cooled scramjet to reduce the fuel flow for cooling. Fuel heat sink (cooling capacity) is repeatedly used to indirectly increase the fuel heat sink. Parametric sensitivity analysis of Re-Cooled Cycle of a hypersonic aircraft is explored. An analytical fin-type model for incompressible flow in smooth-wall rectangular ducts in terms of hydrodynamic, thermal, power balance and Mach number constraints is proposed. Based on this model, the difference of the cooling channel structure design between Re-Cooled Cycle and regenerative cooling is discussed, and a new optimization index is introduced for Re-Cooled Cycle. The sensitivity of the cycle performance to cooling channel geometry is investigated, and the optimal performance of a Re-Cooled Cycle is obtained by satisfying constraints. The differences of the effect of channel design variables between Re-Cooled Cycle and regenerative cooling are also discussed.     

Technical Note Flow characteristics in a heated rotating straight pipe

The flow in a rotating or in a heated straight pipe has been extensively studied not only for academic interest, but also for the great importance in mechanical applications such as in pipe heat exchangers, in cooling systems of rotor blades in gas turbines and in chemical mixing. The viscous flow in a straight pipe rotating about an axis perpendicular to its own, has as a result the generation of a secondary flow that is sustained by the Coriolis force introduced by the rotation of the pipe. Barua [1]used a regular perturbation about the Poiseuille flow limit, similar to Deans 2 and 3approach for stationary and curved pipe flow. He showed that rotation generates a secondary flow and that it depends on the non-dimensional parameter R_r = (2Ωα²?ν), where Ω is the angular frequency of rotation, α is the radius of the pipe and ν is the kinematic viscosity. Subsequent boundary-layer analysis also predicted a significant increase in the friction factor with rotational speed for small rotational rates and high axial pressure gradients (Mori and Nakayama [4], Ito and Nanbu [5]), the latter group also obtaining satisfactory agreement with their experimental results. Mansour [6]considered higher rotational velocities using a computer extension for the perturbation expansion, similar to a method that was applied by Van Dyke 7, 8 and 9who studied the flow in a stationary straight pipe. According to this method the equations of motion are modified, so that they are depend on a single parameter K = R_eR_r, under the assumption that R_e → ∞, R_r → 0, where R_e is the Reynolds number based on the axial velocity. Benton [10]considered small rotational velocities and constructed a small perturbation expansion about the Hagen–Poisseuille flow. Later, Benton and Boyer [11]assumed the case of a rapid rotating conduit with R_rR_e ≤ 1. Duck [12]used a numerical procedure, based on a combination of Fourier decomposition and finite difference discretization to study the flow structure in rotating circular ducts.The first experiments concerning a rotating pipe were conducted by Trefethen [13]who observed that rotation transfers the onset of turbulence to higher Reynolds numbers. Later, Euteneuer and Piesche [14]in their experimental studies in circular pipes confirmed that the pressure drop is significantly higher than that for straight pipes, in agreement with the theoretical results.The earliest analysis on the flow in a heated straight pipe was considered by Morton [15]. His study was restricted to small rates of heating and he obtained solutions for the axial velocity and temperature as power series depending on the parameters R_eR_a, where R_a is the Rayleigh number based on the temperature gradient along the pipe wall. Mori and Nakayama [16]assumed velocity and temperature boundary layers along the pipe wall and analysed theoretically the flow field and the temperature field. Van Dyke [17]modified Mortons variable in order to clarify the dependence of the problem on the parameters P_r, R_a and R_e, where P_r is the Prandtl number. The advantage of these simplifications was that the flow depended only on two parameters ε = P_rR_aR_e and P_r, respectively. Guiasu et al. [18]were able to compute many terms of the series on Mortons problem using symbolic computation packages.In the present work we study the fully developed steady flow in a straight rotating heated pipe with circular cross-section. The equations of motion and energy depend on three parameters that characterize the flow, the rotational Reynolds number R_r, the Reynolds number based on the axial flow R_e and the Rayleigh number R_a, and they are solved both analytically and numerically. In the analytical solution the functions of the flow are expanded in power series of the parameters R_r and R_a. Because of the difficulties of the problem introduced by the presence of these three parameters, we were able to compute only ten terms in each series, so that the range of values of the parameters for which the analytical solution converges is limited by the products R_eR_a < 1000 and R_eR_r < 250. However the analytic expression is of some value because it provides us with a mathematical expression showing the trend of the flow characteristics as well as with a benchmark for the numerical solution. In the numerical solution we consider a grid of mesh points in the circular domain and we modify the differential equations by approximating all partial derivatives with central differences. In this way we deduce an algebraic system of equations for all the points of the circular region that is solved using an iterative procedure. The limits of the products R_eR_a and R_eR_r for which the numerical solution is valid are R_eR_a < 20 000 and R_eR_r < 5000. We compared the results obtained by the two methods and finally we examined the influence of Coriolis and buoyancy forces on the flow and the dependence of some properties of the flow, as the axial and azimuthal stresses, the Nusselt number, on the previous products.     

Laminar heat transfer performance of power law fluids in coiled square tube with various configurations

This study addresses heat transfer performance of laminar non-Newtonian fluid flow in various configurations of coiled square tubes e.g., in-plane spiral ducts, helical spiral ducts and conical spiral ducts. The non-Newtonian fluid considered in this study is the aqueous solution of carboxymethyl cellulose (CMC) which is modeled as power-law fluid. Effects of tube geometries, power-law index (concentration of CMC) and other parameters are quantified and discussed to analyze flow behavior and heat transfer performance. Results are compared with those for a straight square tube of the same length as that used to form the coils. A Figure of Merit is defined to compare the heat transfer performance of different geometries with respect to the pumping power. The results suggest that CMC solution yields better heat transfer performance of about twice than that of water at Re ~ 1000. Among all considered designs, helical coil gives the best heat transfer performance; however, when the pumping power is considered, in-plane coil design performs the best in term of Figure of Merit.     

Optimization of a central-heating radiator

An approximate analytical model has been used to evaluate the optimum dimensions of a central-heating radiator. The radiator problem is divided into three one-dimensional fin problems and then the temperature distributions within the fins and heat-transfer rate from the radiator are obtained analytically. The optimum geometry maximizing the heat-transfer rate for a given radiator volume and the geometrical constraints associated with production techniques, and thermal constraints have been found. The effects of geometrical and thermal parameters on the radiator’s performance are presented.     

Adjoint-Based Geometrically Constrained Aerodynamic Optimization of a Transonic Compressor Stage

Efficient method to handle the geometric constraints in the optimization of turbomachinery blade profile is required. Without constraints on the blade thickness, optimal designs typically yield thinner blade to reduce the friction loss, however, at the risk of degraded strength and stiffness. This issue is seldom discussed and existing literature always treat the blade thickness constraint in an indirect manner. In this work, two different geometric constraints on the blade thickness are proposed and applied in the adjoint optimization: one is on the maximum blade thickness and the other is on the blade area. Methods to compute sensitivities of both constraints are proposed and they are integrated into an optimization system based on a finite volume code and a solver for the discrete adjoint equation. Adjoint optimization is conducted to minimize the entropy production in a transonic compressor stage. Results for the adjoint optimization without geometry constraint and two comparative cases are detailed. It is indicated that three cases yield similar performance improvement; however, if geometry constraints are properly handled, the optimal designs have almost the same maximum thickness as the original design, compared to a thinner blade profile with 14% reduction of maximum thickness in the case without geometry constraint. The cases considering geometry constraints also consume slightly reduced Central Processing Unit(CPU) cost. Result of this work verifies the effectiveness of the proposed method to treat geometric constraints in adjoint optimization.     

Thermodynamic optimization of tree-shaped flow geometries

In this paper we optimize the performance of several classes of simple flow systems consisting of T- and Y-shaped assemblies of ducts, channels and streams. In each case, the objective is to identify the geometric configuration that maximizes performance subject to several global constraints. Maximum thermodynamic performance is achieved by minimization of the entropy generated in the assemblies. The boundary conditions are fixed heat flow per unit length and uniform and constant heat flux. The flow is assumed laminar and fully developed. Every geometrical detail of the optimized structure is deduced from the constructal law. Performance evaluation criterion is proposed for evaluation and comparison of the effectiveness of different tree-shaped design heat exchangers. This criterion takes into account and compare the entropy generated in the system with heat transfer performance achieved.     

Comparative performance studies of a 4-stroke CI engine operated on dual fuel mode with producer gas and Honge oil and its methyl ester (HOME) with and without carburetor

In order to meet the energy requirements, there has been growing interest in alternative fuels like biodiesels, methyl alcohol, ethyl alcohol, biogas, hydrogen and producer gas to provide a suitable diesel oil substitute for internal combustion engines. Biomass is basically composed of carbon, hydrogen and oxygen. A proximate analysis of biomass indicates the volatile matter to be between 60–80% and 20–25% carbon and the rest, ash. The first part of sub-stoichiometric oxidation leads to the loss of volatiles from biomass and is exothermic; it results in peak temperatures of 1400–1500 K and generation of gaseous products like carbon monoxide, hydrogen in some proportions and carbon dioxide and water vapor, which in turn are reduced in part to carbon monoxide and hydrogen by the hot bed of charcoal generated during the process of gasification. Therefore, solid biomass can be converted into a mixture of combustible gases, and subsequently utilized for combustion in a CI engine. Producer gas, if used in dual fuel mode, is an excellent substitute for reducing the amount of diesel consumed by the CI engine. Downdraft moving bed gasifiers coupled with an IC engine are a good choice for moderate quantities of available biomass, up to 500 kW of electric power. Vegetable oils present a very promising alternative to diesel oil since they are renewable and have similar properties. Vegetable oils offer almost the same power output with slightly lower thermal efficiency when used in diesel engines [1], [2], [3], [4], [5], [6], [7]. Research in this direction with edible oils have yielded encouraging results, but their use as fuel for diesel engines has limited applications due to higher domestic requirement [8], [9], [10]. In view of this, Honge oil (Pongamia Pinnata Linn) is selected and its viscosity is reduced by the transesterification process to obtain Honge oil methyl ester (HOME). Since vegetable oils produce higher smoke emissions, dual fuel operation could be adopted in order to improve their performance. A gas carburetor was suitably designed to maximize the engine performance in dual fuel mode with Honge oil–producer gas and HOME–producer gas respectively. Thus bio-derived gas and vegetable oil, when used in a dual fuel mode with carburetor, resulted in better performance with reduced emissions.     

Towards maximal heat transfer rate densities for small-scale high effectiveness parallel-plate heat exchangers

This paper addresses the question to what extent parallel-plate heat exchangers can be downsized without loss of thermal-hydraulic performance. It is shown that when the characteristic length scales of the channels are reduced at a constant pressure drop, the effectiveness exhibits a maximum due to axial heat conduction. The point of maximal effectiveness is found to correspond to a maximal thermal power density and thus to the minimal volume required for obtaining that effectiveness. Based on asymptotic relations for the effectiveness in the small and large channel limit, closed-form expressions are derived for the optimum geometric parameters that maximize power density in the limit of design effectiveness approaching unity. These relations are extended to a broader effectiveness range by means of dimensionless correction functions that are calculated numerically. The resulting expressions define optimal elemental units that can be used to construct parallel-plate counter-flow heat exchangers with the lowest possible core volume for effectiveness values between 0.53 and 1.     

Modeling and optimization of designing parameters for a parallel-plain fin heat sink with confined impinging jet using the response surface methodology

The parallel-plain fin (PPF) array structure is widely applied in convective heat sinks in order to create extended surface for the enhancement of heat transfer. In the present study, for investigating the influences of designing parameters of PPF heat sink with an axial-flow cooling fan on the thermal performance, a systematic experimental design based on the response surface methodology (RSM) is used. The thermal resistance and pressure drop are adopted as the thermal performance characteristics. Various designing parameters, such as height and thickness of fin, width of passage between fins, and distance between the cooling fan and the tip of fins, are explored by experiment. Those parameters affect the structure arrangement, geometry of fins and the status of impinging jet from an axial-flow cooling fan installed over the heat sink. A standard RSM design called a central composite design is selected as experimental plan for the four parameters mentioned above. An effective procedure of response surface methodology (RSM) has been proposed for modeling and optimizing the thermal performance characteristics of PPF heat sink with the design constrains. The most significant influential factors for minimizing thermal resistance and pressure drop have been identified from the analysis of variance. The confirmation experimental results indicate that the proposed model is reasonably accurate and can be used for describing the thermal resistance and pressure drop with the limits of the factors studied. The optimum designing parameters of PPF heat sink with an axial-flow cooling fan under constrains of mass and space limitation, which are based on the quadratic model of RSM and the sequential approximation optimization method, are found to be fin height of 60 mm, fin thickness of 1.07 mm, passage width between fins of 3.32 mm, and distance between the cooling fan and the tip of fins of 2.03 mm.     

Thermodynamic optimization of geometry: T- and Y-shaped constructs of fluid streams

This paper presents a series of examples in which the global performance of flow systems is optimized subject to global constraints. The flow systems are assemblies of ducts, channels and streams shaped as Ts, Ys and crosses. In pure fluid flow, thermodynamic performance maximization is achieved by minimizing the overall flow resistance encountered over a finite-size territory. In the case of more complex objectives such as the distribution of a stream of hot water over a territory, performance maximization requires the minimization of flow resistance and the leakage of heat from the entire network. Taken together, these examples show that the geometric structure of the flow system springs out of the principle of global performance maximization subject to global constraints. Every geometric detail of the optimized flow structure is deduced from principle. The optimized structure (design, architecture) is robust with respect to changes in some of the parameters of the system. The paper shows how the geometric optimization method can be extended to other fields, e.g., urban hydraulics and, in the future, exergy analysis and thermoeconomics.     

Geometric optimization of morphing fins coupled with a semicircular heat generating body: A numerical investigation on the basis of Bejan's theory

The objective of the present work is to optimize, by means of constructal design associated with exhaustive search and genetic algorithm, the geometry of morphing T-shaped fins that remove heat from a semicircular basement. The fins are bathed by a steady stream with constant ambient temperature and convective heat transfer. The semicircular body that serves as a basement for the T-shaped construct generates heat uniformly and it is perfectly insulated on the outer perimeter. It is shown numerically that the global thermal resistance can be minimized by geometric optimization subjected to constraints, namely, the basement area constraint, the T-shaped fins area fraction constraint and the auxiliary area fraction constraint, i.e. the ratio between the area that circumscribes the T-shaped fin and the basement area. The combination of the degrees of freedom values in the context of constructal design generated a search space with several “potential” local minima so that the classic technique, i.e. the exhaustive search, had to be substituted by the genetic algorithm method. In this context, the initial investigation regarding the degrees of freedom L₁/L₀ and t₁/t₀ was performed by means of the exhaustive search, while the parameters k_p, ϕ, λ and ψ have been studied by employing GA technique. First achieved results indicate that when the geometry is free to morph then the thermal performance is improved according to the constructal principle named by Bejan “optimal distribution of imperfections”. Finally, a comparative analysis between T-shaped constructs coupled with rectangular, trapezoidal and semicircular geometries has been carried out in terms of effectiveness in heat removal. The performance of the T-shaped morphing fin having semicircular basement (the case here treated) proved to be considerably superior than the other tested geometries.     

CIRCUMFERENTIAL CRACK PROBLEM FOR AN FGM CYLINDER UNDER THERMAL STRESSES

The main objective of this study is to determine the stress intensity factors associated with a circumferential crack in a thin-walled cylinder subjected to quasi-static thermal loading. The cylinder is assumed to be a functionally graded material. In order to make the problem analytically tractable, the thin-walled cylinder is modeled as a layer on an elastic foundation whose thermal and mechanical properties are exponential functions of the thickness coordinate. Hence a plane strain crack problem is obtained. First temperature and thermal stress distributions for a crack-free layer are determined. Then using these solutions, the crack problem is reduced to a local perturbation problem where the only nonzero loads are the crack surface tractions. Both internal and edge cracks are considered. Stress intensity factors are computed as functions of crack geometry, material properties, and time.