Validation of CFD simulation for flat plate solar energy collector

The problem of flat plate solar energy collector with water flow is simulated and analyzed using computational fluid dynamics (CFD) software. The considered case includes the CFD modeling of solar irradiation and the modes of mixed convection and radiation heat transfer between tube surface, glass cover, side walls, and insulating base of the collector as well as the mixed convective heat transfer in the circulating water inside the tube and conduction between the base and tube material. The collector performance, after obtaining 3-D temperature distribution over the volume of the body of the collector, was studied with and without circulating water flow. An experimental model was built and experiments were performed to validate the CFD model. The outlet temperature of water is compared with experimental results and there is a good agreement.     

CFD analysis of mist/air film cooling on a flat plate with different hole types

The flow and heat transfer mechanisms of mist/air film cooling are studied in this paper with three hole types under three blowing ratios. The velocity vectors and three-dimensional contours of the vortices are shown in this paper in order to investigate the mist/air cooling characters with different hole types. Both the span averaged and centerline film cooling effectiveness are studied in this paper to compare the cooling performance of three cooling hole types.

Mist/air cooling performances of cylindrical hole, fan-shaped hole, and console hole are studied numerically. Flow structures of mist/air jets are studied. Both centerline and span averaged film cooling effectiveness are studied. Results show that the kidney vortex uplifts the mists further away from the flat plate and the antikidney vortex uplifts the mists near the lateral edge of the hole gradually. Mists enhancement on cooling effectiveness is significantly impaired at a higher blowing ratio in cylindrical hole and fan-shaped hole cases. The cooling effectiveness can maintain a relatively high value at each blowing ratio in console hole cases.     

Effects of culture parameters, light, and mixing on H2 production by sulfur-deprived Chlamydomonas reinhardtii in flat plate reactor

Based on advective-diffusive reaction equation for inhomogeneous biochemical system and an empirical equation for light attenuation coefficient, the interplay among culture parameters, light intensity and illumination condition, and mechanical mixing condition during O₂ evolution and H₂ production in a flat plate photobioreactor with sulfur-deprived Chlamydomonas reinhardtii culture is modeled in this work. Four initial chlorophyll concentrations, two light attenuation levels, and two illumination conditions were modeled to study their effects on the dynamics of O₂ evolution and H₂ production. The results indicate that two side illumination is the best design for light penetration into a flat plate reactor. While for single side illumination condition, an optimal combination of the initial cell concentration, light intensity, and reactor width may have to be considered for high H₂ production.     

Entropy analysis of unsteady flow on flat plate

The flow generated due to the motion of flat plate is an important problem in fluid mechanics, since it gives insight into the unsteady boundary layer generation. The entropy analysis of such flow problems can provide enlightening information on the viscous dissipation in the fluid flow. In the present study, an impulsively started Couette flow is studied, where a flow between two parallel plates is moved impulsively by moving the lower plate from rest to a finite velocity in the half‐space y > 0. Development of velocity and entropy profiles, at different time scales, has been numerically obtained; the development of total entropy with time, in the space between the plates, has been determined. It is found that entropy generation in the space between the plates is more considerable at initial times of motion than at later times. Copyright © 2001 John Wiley & Sons, Ltd.     

Characterization of solar flat plate collectors

Characterization of solar collectors is based on experimental techniques next to validation of associated models. Both techniques may be adopted assuming different complexities. In this work, a general methodology to validate a collector model, with undetermined associated complexity, is presented. It serves to characterize the device by means of critical coefficients, such as the film (convection) transfer coefficient, plate absortance or emmitance. The first step consists of identifying those significant parameters that match the selected model with the experimental data, via nonlinear optimization techniques, applied to steady state conditions. Second, new correlations must be adopted, in those terms where it is necessary (i.e. film coefficient equations). Finally, the overall model must be checked in transient regime. To illustrate the technique, a tailor-made prototype flat plate solar collector has been analyzed. An intermediate complex collector model has been proposed (2D finite-difference method). Both steady and transient states were analyzed under different operating conditions. Parameter identification is based on Newton's method optimization. For parameter approximation, exponential regression functions through multivariate analysis of variance is proposed among many other alternatives. Results depicted a robustness of the overall proposed method as starting point to optimize models applied to solar collectors.     

CFD simulation for turbulent mixing with the stochastic approach

A computational fluid‐dynamic simulation for a turbulent nonuniform combustion is established using a stochastic approach. At each point in a turbulent flow field, the variations of species mass fraction and temperature are statistically described by the joint‐probability density function (pdf), and the velocity variation is expressed using the conventional k–? turbulent model. The transport equation of this joint pdf of mass fraction and temperature is calculated by a finite‐difference method in convection and turbulent diffusion and by the Curl collision‐redispersion model in molecular mixing. This method is applied to simulate the process of scalar dispersion in a uniform isotropic turbulent flow. The results show that the profile of an averaged scalar is quite similar to those calculated using conventional transport equations. Furthermore, a reasonable degree of reproduction is achieved for the pdfs of the scalar at each point in the flow field. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 503–511, 2001     

Effect of concentration on the performance of flat plate photovoltaic modules

The effect of low concentration ratios on the performance of passively cooled conventional photovoltaic modules has been investigated. Peak power outputs of up to 140 W per square metre of module area have been obtained with single crystal modules of high cell packing factor using a 2.2X plane mirror concentrator. Both cell temperature and series resistance losses are found to be important in limiting module efficiency. Performance simulations indicate that the use of a 4X concentrator with polar axis tracking will increase annual peak output by a factor of 3.2 over that of a fixed flat plate module.     

平板太阳集热器防冻结构试验研究

提出利用预留空间结构的防冻方案,并取得了初步研究成果。     

CFD analysis of the thermal losses on the upper part of a flat solar collector

To investigate the reduction of heat losses on the upper part of a flat solar collector, a two‐dimensional study was carried out by CFD analysis using Fluent. For this purpose, the heat transfer behavior in the air gap over a wide range of thicknesses of the latter (1‐20 mm) and the addition of a second glass cover fixed at midheight of the air gap spacing have been investigated. For small thicknesses of the air gap (1‐8 mm), the heat transfer is essentially conductive. An increase in the thickness leads to the intensification of the natural convection which induces high thermal losses. The simulation results have shown that the addition of a second cover glazing leads to the weakening of the natural convection and thus to an average enhancement of the solar collector temperature over the range of thicknesses studied of approximately 17%. The overall thermal losses coefficient is then reduced by an average of 26% compared with the single‐glazed solar collector. They have also shown that the thickness of the air gaps resulting in the minimum overall heat losses is 8 mm and that the thickness of the second glass cover has no significant effect on these results. In addition, this study has highlighted the importance of taking into account the radiation heat transfer in establishing the thermal balance of a flat solar collector. Indeed, this consideration leads to an average decrease of the absorber temperature of about 30%.     

Performance of suspended flat plate air heater

In this communication, a heat transfer model to predict the transient behaviour of a suspended flat plate solar collector with constant flow of fluid (air) above the absorber has been presented. A reflecting sheet with an air gap between the absorber plate and bottom insulation to reduce heat loss has been used. The effect, on performance of the air heater, of the parameters viz, spacing between cover and plate, heat capacity of air and absorber plate, flow rate of fluid and collector length have been studied. The effect of changing the averaging inlet temperature with varying collector length has also been studied.     

Exergetic optimization of flat plate solar collectors

In this paper, an exergetic optimization of flat plate solar collectors is developed to determine the optimal performance and design parameters of these solar to thermal energy conversion systems. A detailed energy and exergy analysis is carried out for evaluating the thermal and optical performance, exergy flows and losses as well as exergetic efficiency for a typical flat plate solar collector under given operating conditions. In this analysis, the following geometric and operating parameters are considered as variables: the absorber plate area, dimensions of solar collector, pipes' diameter, mass flow rate, fluid inlet, outlet temperature, the overall loss coefficient, etc. A simulation program is developed for the thermal and exergetic calculations. The results of this computational program are in good agreement with the experimental measurements noted in the previous literature. Finally, the exergetic optimization has been carried out under given design and operating conditions and the optimum values of the mass flow rate, the absorber plate area and the maximum exergy efficiency have been found. Thus, more accurate results and beneficial applications of the exergy method in the design of solar collectors have been obtained.     

Fluid oscillations in flat plate boiling water collectors

Boiling water collectors with an internal two-phase thermosiphon and buoyancy driven circulation are apt for process steam generation in the temperature range 70–150°C. Exploiting the advantages of boiling heat transfer requires mastering the instabilities of two-phase flow natural convection. With electrically heated mock-up test devices the boundaries for stable flow conditions were investigated. Between these boundaries, periodic mass flow fluctuations occur and may stimulate resonances of collector components. The fluctuation frequencies measured with the collector mock-up were below 1 Hz. The origin of the oscillations is still under investigation.     

Study of transmittance in an evacuated flat plate collector with double cylindrical glass cover

A generalized theory for the computation of transmittance of a flat plate collector with double cylindrical cover is given. Effective transmittance for the collector at different azimuths and different orientations has been computed. This study gives an idea about collector performance at different orientations on a whole day basis.     

蜂窝热管平板式太阳能热水器的实验研究

把蜂窝技术和热管技术结合起来应用于平板式太阳能热水器，可以大大减少热损失，提高日平均效率。经多次实验证明，蜂窝热管平板式太阳能热水器的日平均效率可以比真空管式热水器的高，热损系数则要小很多，具有推广应用价值。     

Forced convection of non-Newtonian fluids on a heated flat plate

Forced convective heat transfer due to a non-Newtonian fluid flowing past a flat plate has been investigated using a modified power-law viscosity model. This model does not contain physically unrealistic limits; consequently, no irremovable singularities are introduced into boundary-layer formulations for such fluids. Therefore, the boundary-layer equations can be solved by (numerically) marching downstream from the leading edge as is common for boundary layers involving Newtonian fluids. For shear-thinning and shear-thickening fluids, non-Newtonian effects are illustrated via velocity and temperature distributions, shear stresses, and heat transfer rates. The most significant effects occur near the leading edge, gradually tailing off far downstream where the variation of shear stresses becomes smaller.