Adiabatic Extensions Research Articles

Improved cooling of photovoltaic panels by natural convection flow in a channel with adiabatic extensions.

In hot dry regions, photovoltaic modules are exposed to excessive temperatures, which leads to a drop in performance and the risk of overheating. The present numerical study aims to evaluate the natural air cooling of PV modules by an inclined chimney mounted at the back. The basic equations were solved using the finite volume method. The validity of the model is verified by comparison with the data available in the literature. Thermal and dynamic flow patterns are analyzed for a variety of parameters: Rayleigh numbers from 102 to 106, PV panel tilt angle from 15° to 90°, and channel aspect ratios from 1/20 to 1/5. A critical aspect ratio has been determined to minimize overheating of the PV module. According to the computational results, the tilt angle and modified Rayleigh number increase the mass flow rate and mean Nusselt number. The overheating zone with maximum temperatures is located in the upper part of the photovoltaic panel. The addition of an extension to both channel's inlet and outlet was found to improve the cooling of the photovoltaic panels; however, only the extensions downstream of the channel are truly effective. The critical lengths at which channel performance improves significantly were identified by examining the impact of longer extensions on channel performance. Increasing the extension length from 0 to 3H improves the mass flow rate by 65%, the average Nusselt number by 13.4%, and leads to an 11% decrease in maximum temperature when Ra* = 106. This cooling technique is particularly promising for hot dry regions where water is scarce.

Geometry contributions to natural convection heat transfer enhancement by the chimney effect

Natural convection heat transfer enhancement continues to be an evolving problem in the design of passive safety systems in nuclear engineering. Increasing heat rejection rates guarantees safety and higher heat duty. This study numerically investigated the influence of the geometry of an adiabatic extension appended downstream of a vertical heated cylinder. Four geometries comprising straight, diffuse, hyperboloid, and compound adiabatic extensions were studied. Nusselt number ratios, temperature distribution ratios, and mass flow rates were presented. Results showed that the compound extension geometry exuded high heat transfer enhancement capabilities by the chimney effect, and reduced wall temperature by up to 16% for the range of parameters studied. These findings are presented as a contribution to the design improvements of air-cooled passive systems, especially in advanced nuclear reactors.

Proposal of a spin-one chain model with competing dimer and trimer interactions

A new kind of spin-1 chain Hamiltonian consisting of competing dimer and trimer projection operators is proposed. As the relative strengths and signs of the interactions are varied, the model exhibits a number of different phases including the gapped dimer phase and the gapless trimer phase with critical correlations described by a conformal field theory with central charge $c=2$. A symmetry-protected topological phase also exists in this model, even though the microscopic interactions are not the simple adiabatic extensions of the well-known Heisenberg and the Affleck-Kennedy-Lieb-Tasaki model and contains both two- and three-particle permutations. A fourth phase is characterized by macroscopically degenerate ground states. While bearing almost a one-to-one resemblance to the phase diagram of the bilinear-biquadratic spin-1 chain Hamiltonian, our model is rooted on very different physical origin, namely the two competing tendencies of spin-1 particles to form singlets through either dimer or trimer formation.

English

The flow field in the hyperbolic natural draft wet-cooling tower, which has great effects on the economy and security of power plant, was studied through numerical simulation. The mathematical model was established and analysed in order to optimize the cooling-tower and to evaluate its efficiency and a Numerical study on a channelchimney system was carried out in order to elucidate the behaviour of heat transfer and fluid flow. A 2D CFD model of natural draught Cooling tower and chimney like heat sink plate is developed the Euler multiphase model was used to simulate the heat and mass transfer in a natural draught cooling tower rain zone and the RNG turbulent model with the dispersed option is used for the turbulence modeling of multiphase flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions.

Effects of adiabatic extensions on heat transfer through a differentially heated square cavity

This study is concerned with transient and steady state heat transfer by natural convection in a differentially heated cavity. The purpose is to evaluate a passive approach for enhancing heat transfer through the cavity. In this study, the effects of three different corner geometries (including sharp, round and straight corners) and adiabatic extensions of various dimensions on natural convection heat transfer are investigated numerically. The numerical results show marginal variations of the heat transfer rates among the three different corner shapes and a strong dependence of heat transfer enhancement on the Rayleigh number. For a given Rayleigh number, the enhancement of heat transfer by adiabatic extensions is limited.

Natural Convection in Vertical Channels with Porous Media and Adiabatic Extensions

A numerical investigation on natural convection in air in a vertical heated channel, partially filled with porous medium, with adiabatic extensions downward and collinear the heated plates is accomplished. The fluid flow is assumed two-dimensional, laminar, steady state and incompressible. The porous material is considered as homogeneous and isotropic and the Brinkman-Forchheimer-extended Darcy model is considered. A finite-extension computational domain is employed to simulate the free-stream condition and allows to account for the diffusive effects and the numerical results are obtained using the finite volume method by FLUENT. Results in terms of wall temperature profiles are presented to evaluate the effects of the main thermal and geometrical parameters. The adiabatic extensions determine a wall temperature decrease and wall temperature decreases increasing Darcy number. In full filled heated channels wall temperature presents a significant increase for Darcy number decrease.

Experimental Investigation of Natural Convection in an Asymmetrically Heated Vertical Channel with an Asymmetric Chimney

An experimental investigation on air natural convection, in a vertical channel asymmetrically heated at uniform heat flux, with downstream unheated parallel extensions, is carried out. One extension is coplanar to the unheated channel wall and the distance between the extensions is equal to or greater than the channel gap (geometrically asymmetric chimney). Experiments are performed with different values of the wall heat flux, aspect ratio (Lh∕b), extension ratio (L∕Lh) and expansion ratio (B∕b). For the largest value of the aspect ratio (Lh∕b=10), the adiabatic extensions improve the thermal performance in terms of lower maximum wall temperature of the channel. Optimal configurations of the system with asymmetrical chimney are detected. Flow visualization shows a cold inflow in the channel-chimney system that penetrates down below the channel exit section. Maximum wall temperatures and channel Nusselt numbers are correlated to the channel Rayleigh number, Ra*, and to the geometrical parameters, in the ranges 3.0×102⩽Ra*B∕b⩽1.0105, 1.0⩽B∕b⩽3.0 and 1.0⩽L∕Lh⩽4.0 with Lh∕b=5.0 and 10.0.

Heat and fluid flow resulting from the chimney effect in a symmetrically heated vertical channel with adiabatic extensions

An experimental study on a channel-chimney system was carried out in order to elucidate the behavior of heat transfer and fluid flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions. Different fluid motion regions are observed inside the chimney. Inflows of air are detected in the lower extension ratio, particularly for large values of the ratio of the width of chimney to that of the heated channel. Some typical configurations show the presence of a vortex structure for an expansion ratio greater than one close to the corner regions in the chimney. Some monomial correlation equations between the local Nusselt number, the channel Rayleigh number and the geometric parameters are proposed. The dimensionless parameters are in the following ranges: 10 2⩽Ra ∗(B/b)⩽10 6; 1.5⩽L/L h⩽4.0; 1.0⩽B/b⩽4.0 , in which L is the total height of the system, L h is the height of the heated channel, B is the width of the chimney and b is the width of the heated channel. A good agreement between the correlation and the experimental data is observed.

Heat transfer enhancement by the chimney effect in a vertical isoflux channel

The ease of thermal control by means of air natural convection stimulates the investigation of configurations with the aim at improving the thermal performance. The effect of adding adiabatic extensions downstream of a vertical isoflux symmetrically heated channel has been experimentally analyzed. Optimal configurations have been identified through the measured wall temperature profiles, with reference to the extension and expansion ratios ( L/ L h and B/ b) of the insulated extensions. Conspicuous maximum wall temperature reductions have been achieved by means of these optimal configurations. In details, percent increases of the heat transfer rate (i.e., average channel Nusselt number) were of order 10–20% depending on the channel aspect ratio, L h/ b, and imposed wall heat flux. In any case, quite large extensions should be added to enhance the heat transfer rate, i.e. about 3.0 times the height of the channel, while the optimal expansion ratio was nearly 2.0 for all the configurations. Composite correlations between the average Nusselt number and the maximum dimensionless wall temperatures and Ra *, the Rayleigh channel number, B/ b and L/ L h parameters, in the 1.5⩽ L/ L h⩽4.0,1.0⩽ B/ b⩽4.0 and 10 2⩽ Ra * B/ b⩽5.0×10 6 ranges, have been evaluated.

Adiabatic diffusion Monte Carlo approaches for studies of ground and excited state properties of van der Waals complexes

Several adiabatic extensions to the diffusion Monte Carlo approach are presented. In the first, an adiabatic form of the finite field method is developed for the systematic evaluation of expectation values. In addition, an adiabatic flexible node method for calculating excited states is described. The above methods are applied to NeSH and Ar2HCl where comparisons to results of variational calculations can be made.

Natural convection below heated horizontal rectangular plates

The boundary layer equations describing the high Grashof number laminar natural convection flow beneath a heated horizontal surface are solved numerically for an infinitely long strip with a uniform temperature central region and horizontal adiabatic extensions on the sides, and for rectangular plates of two different aspect ratios kept at constant temperature. The boundary condition required by these equations at the edge of the plate is discussed. The results are compared with existing approximate theoretical results and with experiments, as well as with the solutions obtained previously using the same method for a strip and a circular disk.

Heat Transfer Enhancement From a Vertical, Isothermal Channel Generated by the Chimney Effect

A numerical and experimental investigation of free convection from vertical, isothermal, parallel-walled channels has been undertaken to explore the heat transfer enhancement obtained by adding adiabatic extensions of various sizes and shapes. Investigations were carried out for air (Pr= 0.7) over a wide range of wall heating conditions. In all cases, the adiabatic extensions were able to increase heat transfer. The increase varied from 2.5 at low Ra* to 1.5 at high Ra*. The experimental and numerical results are in excellent agreement. A single correlation accounting for the channel aspect ratio Lh/b, expansion ratio, B/b, modified Rayleigh number, Ra* and heated length ratio, Lh/L is presented.

Edge and aspect ratio effects on natural convection from the horizontal heated plate facing downwards

Results of an experimental study of natural convection from a downward facing horizontal heated plate are reported. Measurements were made for square and rectangular plates in air, water and a high Prandtl number oil. The plates were used with bare edges and with approximately adiabatic extensions around the edges. An explanation and correlation of the edge effects is made in terms of displacing the origin of a boundary layer solution. The correlation so obtained accounts for bare edges or adiabatic extensions, and plate aspect ratio, unlike those previously available.