Teaching Documents by taoufik brahim
= = Re < 5.10 5 et 10 ≥ Pr ≥ 0,5 Ecoulement dans un tube Ecoulement turbulent : Nu = 0,023 Re 0,8... more = = Re < 5.10 5 et 10 ≥ Pr ≥ 0,5 Ecoulement dans un tube Ecoulement turbulent : Nu = 0,023 Re 0,8 Pr n n = 0,3 si θ fluide > θ paroi n = 0,4 si θ fluide < θ paroi Re calculé pour D H = 4S / P où : S = section de passage du fluide P = périmètre de contact fluide/paroi
Papers by taoufik brahim
Energy and exergy analysis of a long-term nonlinear dynamic roll bond PVT solar collector model under Tunisian (North Africa) climatic conditions
Thermal science and engineering progress, Jul 1, 2024
Theoretical and experimental investigation of plate screen mesh heat pipe solar collector
Energy Conversion and Management, Nov 1, 2014
ABSTRACT Heat pipes are efficient heat transfer devices for solar hot water heating systems. Howe... more ABSTRACT Heat pipes are efficient heat transfer devices for solar hot water heating systems. However, the effective downward transfer of solar energy in an integrated heat pipe system provides increased design and implementation options. There is a lack of literature about flat plate wicked assisted heat pipe solar collector, especially with the presence of finned water-cooled condenser wicked heat pipes for solar energy applications. In this paper the consequence of incorporating fins arrays into the condenser region of screen mesh heat pipe solar collector is investigated. An experimental and a transient theoretical model are conducted to compare the performances of solar heating system at different period of the year. A good agreement is shown between the model and the experiment. Two working fluids are investigated (water and methanol) and results reveal that water slightly outperforms methanol with a collector instantaneous efficiency of nearly 60%. That modest improvement is achieved by adding fins to the condenser region of the heat pipes. Results show that the collector efficiency increase as the number of fins increases (upon certain number) and reveal that the mesh number is an important factor which affect the overall collector efficiency. An optimal heat pipe mesh number of 100 meshes/in. with two layers appears to be favorable in such collectors for their design and operating conditions.
Discussion on the stability of nanofluids for optimal thermal applications
Elsevier eBooks, 2024
Comparative study of parabolic trough solar collector using sysltherm-800 and therminol-VP1 non-metallic nanofluids
Thermal Science and Engineering Progress
A Novel Approach for Performance Analysis of Air–Air Wickless Heat Pipe Heat Exchanger
Journal of Thermal Science and Engineering Applications, 2022
Wickless heat pipe heat exchangers (HPHE) consistently showed instabilities and omits many experi... more Wickless heat pipe heat exchangers (HPHE) consistently showed instabilities and omits many experimental tests. The performance of a heat pipe is governed by many parameters, and the effects of which may influence each other. The objective of this paper is to develop a new approach for an air–air heat pipe heat exchanger that takes into consideration the effect of heat transfer coefficients, saturation temperature, and thermal resistances inside the heat pipes as well as maximum heat transfer limit. The approach is based on analyzing inner heat pipe parameters with HPHE external working conditions. Results are also assessed for higher heat capacities ratio and found that the thermal resistances inside a heat pipe are a limiting factor, leading the HPHE system to perform poorly in particular for Cr ≤ 1. Overall heat transfer coefficients at HPHE sides as well as HPHE effectiveness as a function of Cr are assessed. Trying to follow the Chaudourne (1992, “The Heat Pipe Heat Exchangers: ...
Feasibility Study of Long-Term Dual Tank Photovoltaic/Thermal Indirect Parallel Solar-Assisted Heat Pump Systems
Journal of Solar Energy Engineering, 2022
A novel dual tank photovoltaic/thermal (PV/T) indirect parallel solar-assisted heat pump system w... more A novel dual tank photovoltaic/thermal (PV/T) indirect parallel solar-assisted heat pump system was investigated in this paper, which filled a gap in the literature. Furthermore, a long-term performance study analysis was performed under Tunisian climate to offset domestic electric and hot water loads. Optimal operations of such a system are achieved based on a simplified mathematical model. Results showed that the average thermal and electric energy efficiency is about 39.65% and 11.38%, respectively. Results revealed that the increase in solar radiation results in an improvement of the system's thermal-based coefficient of performance efficiency coefficient reaching 4.49 at 893 W/m2. PV/T average electrical energy output is found to 0.68 kW h/m2/day with an annual average of 177.42 kW h/m2, which leads to an annual electricity surplus of about 5.83%. A reversible heat pump operation seemed more advantageous especially in the summer months, reducing yearly electric demand by ab...
A transient analysis has been carried out in detail using the finite difference method for the ro... more A transient analysis has been carried out in detail using the finite difference method for the roll heat pipe wish is firstly studied by Jalilvand [1] in photocopy machines. The objective of this study is to better understand this type of heat pipes for further heat recovery applications or other designs processes that are essentially transient. The numerical results shows that the roll heat pipe perform better than the annular heat pipe of same dimension and the condenser region is essentially controlled by the heat Input and the convective heat transfer coefficient. .
Parametric study of photovoltaic/thermal wickless heat pipe solar collector
Energy Conversion and Management, 2021
Abstract Wickless heat pipes are an effective passive heat transfer device that, due to their imp... more Abstract Wickless heat pipes are an effective passive heat transfer device that, due to their improved heat transfer capabilities, can enhance photovoltaic system efficiency. A new photovoltaic-thermal acetone wickless heat pipe solar panel (PVT/WHP) is described in this study. The main parameters affecting the thermal and electrical efficiency of the solar panel, such as wind velocity, incident radiation, water inlet temperature, heat pipe number, and collector surface area, as well as inner heat pipe behavior and operating limits, are studied using a mathematical model. According to the reduced temperature simulation results, the theoretical PVT/WHP module's average electrical, thermal, and overall efficiency were approximately 12.52%, 43.75%, and 56.27%, respectively. The study is being used to assess the thermal efficiency of PVT/WHP under Tunisian climatic conditions, and the simulation results show that PVT/WHP outperforms conventional PVT water-based systems with a maximum thermal and electrical efficiency gain of approximately 21.9% and 14.2% respectively. The current parametric study provided a framework for assessing such a system's behavior and providing useful flexibility to achieve the best possible system performance.
CFD analysis of hotspots copper metal foam flat heat pipe for electronic cooling applications
International Journal of Thermal Sciences, 2021
Abstract Hotspots have become one of the most limiting factors in the conception of more and more... more Abstract Hotspots have become one of the most limiting factors in the conception of more and more performing electronic chips. Although many wicking materials are used as porous media in used as porous media in flat plate heat pipe (FHP), copper metal foams are not well studied with these devices. In the present study, a three-dimensional FHPs with multi heat sources is investigated numerically using COMSOL Multiphysics. The potential use of copper metal foam as a wick structure is investigated. The effect of heat flux, as well as hotspot positions on heat pipe performance, is analyzed and results show that copper metal foam can be an alternative wick used for FHPs. Visualization is made taking into account the hot spot position effect at different heat flux. Results show that top hot spot positions especially close to the wall are the most critical locations where dry out can occur. Hot spot dimensions, as well as their distances and effective pore radius, have a significant effect on heat pipe operation.
Case Studies in Thermal Engineering, 2016
Nanofluids have been receiving increased attention worldwide and various research groups have tri... more Nanofluids have been receiving increased attention worldwide and various research groups have tried to engage it within heat pipes systems to study the subsequent thermal enhancement. The observations based on the reviewed literature showed that the theoretical investigations on nanofluids in heat pipes are very few and hence validating the experimental findings is difficult. In this paper, a two-dimensional numerical model is developed to study the thermal performance of a packed sphere heat pipe utilizing nanofluids due to the recent development of nanofluids and porous media structures. Two of the most common nanoparticles, namely Al 2 O 3 and CuO based water are considered. The substantial change in the heat pipe thermal resistance, liquid pressure, axial and radial velocities profiles in presence of suspended nanoparticles within the base fluid are discussed. The effect of particle size on the thermal performance of the heat pipe is also investigated. Results showed that Al 2 O 3 nanofluid slightly outperformed CuO nanofluid heat pipes especially with decreasing nanofluid particle diameter. It was found that reduction of 68% on thermal resistance can be obtained for 9% CuO nanofluid concentration level which trend to reach 20% without significant shear stress effect. Nanofluid based heat pipe is able to dissipate up to 26% more heat without experiencing an increase in the wall temperature and an optimal nanofluid concentration level and diameter are found to better heat pipe performance.
International Journal of Heat and Technology, 2012
Roll heat pipe is a novel type of heat pipe which is different from conventional or concentric an... more Roll heat pipe is a novel type of heat pipe which is different from conventional or concentric annular heat pipes because heat is transferred radially from the inner pipe (evaporator region) to the outer pipe (condenser region). The condensed liquid flows back from condenser to evaporator by bridge wicks. In this paper, a two dimensional steady state analysis of the performance of the roll heat pipe is reported. The effect of boundary conditions in on the heat pipe thermal resistance is investigated. Result reveal that the RHP thermal resistance decrease with the heat input and the RHP thermal performance is controlled by the outer surface temperature. Result also revealed that a higher grade of isothermalisation can be obtained with heat input location management.
Solar Energy, 2017
In recent years, photovoltaic technology has been developing quickly and studies found that only ... more In recent years, photovoltaic technology has been developing quickly and studies found that only less than 20% solar energy was converted into electricity while more than 50% of the incident solar radiation is transformed into heat. PV module electrical efficiency and operating temperature can be improved by removing the excess heat. This led to the concept of a hybrid collector called PVT collector. Cooling of PV cells has become an important study in order to increase the PV panel efficiency, improvement in power output and optimum performance parameters. Numerous studies theoretical, experimental and economical assessment haves been carried out for different hybrid PVT solar collectors designs. Other alternative conceptions can be improved in different applications. This paper gives a review of the historical and recent trend of PVT development technology, in particular the performance and economic feasibility of PVT system for different application areas. Future recommendations and critical assessment are made to overcome the barriers and challenges focusing of improving PVT technology.
Frontiers in Heat Pipes, 2013
Flow, heat and mass transfer in heat pipes has been studied extensively. This is due to the incre... more Flow, heat and mass transfer in heat pipes has been studied extensively. This is due to the increasing need in understanding the complicated transport process for application of diverse fields. Recently one of the heat pipes which aroused the interest of research are the three dimensional heat pipes which incorporates the phase change mechanisms at the liquid and vapour interface. In this paper, an originally three-dimensional steady state model is developed coupling the hydrodynamic and thermal models in sintered heat pipe in cylindrical polar coordinate. The model predicts the heat pipes behaviors under critical conditions were a high heat input is exposed to its evaporator region. Such study has not been well investigated before and can be useful for academic, engineering and industrial purposes. Compared with its normal working conditions were heat pipes limits are not reached, the results illustrate well the function of the heat pipe and the circulation of the working fluid inside the heat pipe regions. Results show also that, for high heat input the circumferential velocity components increase specially in the liquid-vapor interface and the tangential velocity component has a significant effect on the phase change phenomena.
Parametric Study of a Flat Plate Wick Assisted Heat Pipe Solar Collector
Journal of Solar Energy Engineering, 2013
The use of heat pipes in solar collectors offers several advantages regarding flexibility in oper... more The use of heat pipes in solar collectors offers several advantages regarding flexibility in operation and application, as they are very efficient in transporting heat even under a small temperature difference. Compared with other systems powered by evacuated tube collectors or flat plate solar collectors using a wickless heat pipe, little attention has been paid to a flat plate solar collectors wick assisted heat pipe. In this paper an analytical model based on energy balance equations assuming a steady state condition was developed to evaluate the thermal efficiency of a flat plate wick assisted heat pipe solar collector. Parameters which affect the collector efficiency are identified, such as tube spacing distance, gap spacing between the absorber plate and the glazing cover, and the emissivity of the absorber plate. The results reflect the contribution and significance of each of these parameters to the collector overall heat loss coefficients. Three heat pipe working fluids are...
Journal of Heat Transfer, 2014
The main motivation of conducting this work is to present a rigorous analysis and investigation o... more The main motivation of conducting this work is to present a rigorous analysis and investigation of the potential effect of the heat pipe adiabatic region on the flow and heat transfer performance of a heat pipe under varying evaporator and condenser conditions. A two-dimensional steady-state model for a cylindrical heat pipe coupling, for both regions, is presented, where the flow of the fluid in the porous structure is described by Darcy–Brinkman–Forchheimer model which accounts for the boundary and inertial effects. The model is solved numerically by using the finite volumes method, and a fortran code was developed to solve the system of equations obtained. The results show that a phase change can occur in the adiabatic region due to temperature gradient created in the porous structure as the heat input increases and the heat pipe boundary conditions change. A recirculation zone may be created at the condenser end section. The effect of the heat transfer rate on the vapor radial v...
Numerical investigation of roll heat pipe type for heat exchangers thermal management
Applied Thermal Engineering, 2015
Abstract Heat pipes cooling technology is currently spreading out for many applications such as e... more Abstract Heat pipes cooling technology is currently spreading out for many applications such as electronic, solar energy, air conditioning systems, space applications, telecommunications, food industries, geothermal systems, heat recovery etc. In this work, two numerical studies have been developed to investigate the thermal performance of a novel type of heat pipe called “Roll heat pipe (RHP)”. Results show that RHP performs better than annular pipes often used in heat exchangers. The effect of boundary conditions on the heat pipe thermal resistance is investigated. Results reveal that the RHP thermal resistance decreases with the heat input and RHP thermal performance is controlled by the outer surface temperature. Heat transfer inside RHP can be enhanced when heat is extracted quickly from the condenser region. Result reveals also that a higher grade of isothermalisation can be obtained compared with conventional tubular heat exchanger for uniform or localized heat inputs in the evaporator region. The objective of this study is to better understand the thermal performance of RHP type for further heat exchanger applications or other designs processes that are transient or steady.
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Teaching Documents by taoufik brahim
Papers by taoufik brahim