Showing papers on "Heat transfer coefficient published in 2021"

Convective Heat Transfer Coefficient Model Under Nanofluid Minimum Quantity Lubrication Coupled with Cryogenic Air Grinding Ti–6Al–4V

Abstract: Under the threat of serious environmental pollution and resource waste, sustainable development and green manufacturing have gradually become a new development trend. A new environmentally sustainable approach, namely, cryogenic air nanofluid minimum quantity lubrication (CNMQL), is proposed considering the unfavorable lubricating characteristic of cryogenic air (CA) and the deficient cooling performance of minimum quantity lubrication (MQL). However, the heat transfer mechanism of vortex tube cold air fraction by CNMQL remains unclear. The cold air fraction of vortex tubes influences the boiling heat transfer state and cooling heat transfer performance of nanofluids during the grinding process. Thus, a convective heat transfer coefficient model was established based on the theory of boiling heat transfer and conduction, and the numerical simulation of finite difference and temperature field in the grinding zone under different vortex tube cold air fractions was conducted. Simulation results demonstrated that the highest temperature initially declines and then rises with increasing cold air fraction. Afterward, this temperature reaches the lowest peak (192.7 °C) when the cold air fraction is 0.35. Experimental verification was conducted with Ti–6Al–4V to verify the convective heat transfer coefficient model. The results concluded that the low specific grinding energy (66.03 J/mm3), high viscosity (267.8 cP), and large contact angle (54.01°) of nanofluids were obtained when the cold air fraction was 0.35. Meanwhile, the lowest temperature of the grinding zone was obtained (183.9 °C). Furthermore, the experimental results were consistent with the theoretical analysis, thereby verifying the reliability of the simulation model.

Effect of hybrid nanofluid on heat transfer performance of parabolic trough solar collector receiver

Abstract: In this study, three-dimensional heat transfer and flow characteristics of hybrid nanofluids under turbulent flow condition in a parabolic trough solar collector (PTC) receiver has been investigated. Ag–ZnO/Syltherm 800, Ag–TiO2/Syltherm 800, and Ag–MgO/Syltherm 800 hybrid nanofluids with 1.0%, 2.0%, 3.0%, and 4.0% nanoparticle volume fractions are used as working fluids. Reynolds number is between 10,000 and 80,000. The temperature of the fluid is taken as 500 K. The C++ homemade code has been written for the nonuniform heat flux boundary condition for the outer surface of the receiver. Variations of thermal efficiency, heat transfer coefficient, friction factor, PEC number, Nusselt number, and temperature distribution are presented for three different types of hybrid nanofluids and four different nanoparticle volume fractions with different Reynolds numbers. Also, the graphs of the average percent increase according to Syltherm 800 are given for the working parameters. According to the results of the study, all hybrid nanofluids are found to provide superiority over the base fluid (Syltherm 800) with respect to heat transfer and flow features. Heat transfer augments with the growth of Reynolds number and nanoparticle volume fraction. Thermal efficiency, which is one of the important parameters for PTC, decreases with increasing Reynolds number and increases with the increasing volume fraction of nanoparticle. It is obtained that the most efficient working fluid for the PTC receiver is the Ag–MgO/Syltherm 800 hybrid nanofluid with 4.0% nanoparticle volume fraction.

Application of support vector machines for accurate prediction of convection heat transfer coefficient of nanofluids through circular pipes

Abstract: Convection is one of the main heat transfer mechanisms in both high to low temperature media. The accurate convection heat transfer coefficient (HTC) value is required for exact prediction of heat transfer. As convection HTC depends on many variables including fluid properties, flow hydrodynamics, surface geometry and operating and boundary conditions, among others, its accurate estimation is often too hard. Homogeneous dispersion of nanoparticles in a base fluid (nanofluids) that found high popularities during the past two decades has also increased the level of this complexity. Therefore, this study aims to show the application of least-square support vector machines (LS-SVM) for prediction of convection heat transfer coefficient of nanofluids through circular pipes as an accurate alternative way and draw a clear path for future researches in the field.,The proposed LS-SVM model is developed using a relatively huge databank, including 253 experimental data sets. The predictive performance of this intelligent approach is validated using both experimental data and empirical correlations in the literature.,The results show that the LS-SVM paradigm with a radial basis kernel outperforms all other considered approaches. It presents an absolute average relative deviation of 2.47% and the regression coefficient (R2) of 0.99935 for the estimation of the experimental databank. The proposed smart paradigm expedites the procedure of estimation of convection HTC of nanofluid flow inside circular pipes.,Therefore, the focus of the current study is concentrated on the estimation of convection HTC of nanofluid flow through circular pipes using the LS-SVM. Indeed, this estimation is done using operating conditions and some simply measured characteristics of nanoparticle, base fluid and nanofluid.

Thermophysical properties of graphene-based nanofluids

Abstract: Heat transfer operations are very common in the process industry to transfer a huge amount of thermal energy, i.e., heat, from one fluid to another for different purposes. Many fluids are used as heat transfer fluid (HTF), in which water is the most common HTF due to its high specific heat, availability, and affordability. However, conventional HTFs, including water, have a lower thermal conductivity, which is the most critical thermophysical property, hence decreased heat transfer efficiency. The addition of solid particles of highly thermally conductive material, specifically at nano-size, i.e., nanoparticles NPs, result in nanofluid NF, which has evolved over the last two decades as efficient HTF and have been investigated in a wide range of applications. Among NPs, graphene (Gr) based materials have shown very high potential as NF due to the very high thermal conductivity up to 5,000 W/m.K, hence higher thermal conductivity NF. This work aims to thoroughly discuss the thermophysical properties of Gr-based NFs, including thermal conductivity, heat capacity, density, and viscosity. The discussion focus on the thermophysical properties as it is the ultimate determinator of the heat transfer characteristics of the HTF, such as the convective and the overall heat transfer coefficient as well as the heat transfer capacity of the NF. The discussion expands to the relative enhancement in such thermophysical properties reaching up to a 40% increase in thermal conductivity, as the most critical thermophysical property. The discussion shows that Gr-based NF has a much higher thermal conductivity relative to widely studied metal oxide NF and at much lower content, and lower density and viscosity increase, which is critical for determining the pumping power requirements. Critical challenges facing the application of Gr-based NFs such as cost, stability, increased density and viscosity, and environmental impacts are thoroughly discussed with mitigation recommendations given.

A review on the applications of micro-/nano-encapsulated phase change material slurry in heat transfer and thermal storage systems

Abstract: In modern heat transfer systems, thermal storage not only causes the balance between demand and supply, but also improves the heat transfer efficiency in these systems. In the present study, a comprehensive review of the applications of micro- or nano-encapsulated phase change slurries (MPCMs/NPCMs), as well as their effects on thermal storage and heat transfer enhancement, has been conducted. MPCMs/NPCMs have a myriad of applications and various usages such as pipe and channel flows, photovoltaic/thermal, solar heaters, air conditioning systems, storage tanks and heat pipes that have been categorized and studied. It was found that there are many advantageous adding MPCM/NPCM to the base fluid. The most important effect is that the addition of PCMs to the base fluid can intensify the capacity of energy absorption in the base fluid. These materials can absorb a high proportion of received energy by changing their phase and prevent temperature increment of the base fluid. Thereupon, the specific heat of the fluid in the presence of the micro-/nano-capsules increases. Moreover, in most studies reviewed, heat transfer coefficient and Nusselt number increase by the addition of micro-/nano-capsules to the base fluid. Also, the addition of MPCM/NPCM to the base fluid could make this material pumpable, although increment in the concentration of micro-/nano-capsules raises the viscosity of the working fluid and thereupon the pumping power. On the other hand, for a same heat load, the pumping power decreases due to the lower required flow rate in comparison with pure working fluid. The most important factor that must be considered in the application of MPCMs/NPCMs is the complete phase change of the material. Given the favorable thermal and fluid characteristics of MPCMs/NPCMs, the utilization of these materials could be a promising method to transfer heat and store it with high efficiency and low pumping power.

Development and validation of a TRNSYS type to simulate heat pipe heat exchangers in transient applications of waste heat recovery

Abstract: Heat pipe heat exchangers (HPHEs) are being more frequently used in energy intensive industries as a method of low-grade waste heat recovery. Prior to the installation of a HPHE, the effect of the heat exchanger within the system requires modelling to simulate the overall impact. From this, potential savings and emission reductions can be determined, and the utilisation of the waste heat can be optimised. One such simulation software is TRNSYS. Currently available heat exchanger simulation components in TRNSYS use averaged values such as a constant effectiveness, constant heat transfer coefficient or conductance for the inputs, which are fixed during the entire simulation. These predictions are useful in a steady-state controlled temperature environment such as a heat treatment facility, but not optimal for the majority of energy recovery applications which operate with fluctuating conditions. A transient TRNSYS HPHE component has been developed using the Effectiveness-Number of Transfer Units (ɛ-NTU) method and validated against experimental results. The model predicts outlet temperatures and energy recovery well within an accuracy of 15% and an average of 4.4% error when compared to existing experimental results, which is acceptable for engineering applications.

Thermal performance investigation of concentric and eccentric shell and tube heat exchangers with different fin configurations containing phase change material

Abstract: The aim of this study is to evaluate the combined effect of fin configuration and eccentricity of heat transfer tube on melting behavior of phase change material (PCM) inside the shell and tube heat exchanger (HX). Two unfinned HXs with concentric and eccentric heat transfer tubes and six other HXs with different arrangements of straight and bifurcated fins were fabricated while the total fin mass was kept constant. The shells of heat exchangers were made of transparent Plexiglas and the melting process was photographed to enable the analysis of the solid-liquid interface evolution. Experimental findings of the unfinned HXs showed that applying the eccentric tube HX leads to a 54% reduction in melting time compared to the concentric tube HX. Among all the cases considered, bifurcated fin configuration outperforms the straight fin configuration. The maximum melting time reduction compared to the HX with concentric unfinned tube was 85% obtained by eccentric tube HX with a long upper bifurcated fin and a short lower straight fin. Transient numerical simulations using a control volume approach were carried out for three tube wall temperatures of 75, 85 and 95 °C. Numerical results showed that the bifurcations increase the total heat transfer rate while the convective heat transfer coefficient decreases.

Thermal analysis of a binary base fluid in pool boiling system of glycol–water alumina nano-suspension

Abstract: The main aim of the present research is to measure the nucleate boiling heat transfer coefficient (BHTC) of aqueous glycol nano-suspension as a coolant around a horizontal heater Alumina nanoparticles were added to the base fluid at a volumetric concentration of 1% to improve the thermal conductivity of the nano-suspension The pressure of the system was set to the atmospheric pressure, and the coolant was tested at different applied heat fluxes (HF) ranged from 0 to 90 kW m−2 and volumetric concentration of 0–40% of the heavier component Two zones of heat transfer were identified, including a natural convection zone and nucleate boiling one mixed with bubble formation and bubble interactions Results also showed that the HTC of the nano-suspension is smaller than those recorded for the pure water A rough comparison was made to examine the accuracy of the developed equations for estimating the BHTC value against the experimental data It was found that the developed equations are not accurate for the data measured in the free convection region Thus, the Churchill-Chu correlation was recommended for estimating the BHTC in the free convection area of heat transfer Also, the effect of operating parameters such as HF and volumetric concentration on the pool boiling HTC of the solution was studied

Heat transfer growth of sonochemically synthesized novel mixed metal oxide ZnO+Al2O3+TiO2/DW based ternary hybrid nanofluids in a square flow conduit

Abstract: In the current investigation, the heat transfer development with the turbulent flow of novel metal oxide-based ternary composite nanofluids of ZnO + Al2O3+TiO2/DW at varying wt.% concentrations (0.025, 0.05, 0.075, and 0.1) in a square heat exchanger below constant heat flux conditions was discussed. The new ternary composite nanofluids were synthesized by using the sonochemical technique. The ZnO + Al2O3+TiO2/DW based ternary composite nanofluids with their respective wt. % reveals an enhancement in effective thermal conductivity and heat transfer coefficient local and average with Reynolds numbers varying from 4550 to 20,367. The extreme growth in overall effective thermal conductivity was noticed up to 1.149 W/m-K at 0.1 wt% for ternary hybrid composite nanofluids at a maximum temperature 45 °C. Similarly, at 0.075 wt%, 0.05 wt%, and 0.025 wt% the overall effective thermal conductivity was recorded 1.118 W/m-K, 1.091 W/m-K, and 1.079 W/m-K correspondingly, which is greater than that of base fluid (DW), with improved thermo-physical characteristics for novel ZnO + Al2O3+TiO2/DW ternary hybrid composite nanofluids. Also, it shows an improvement in local and average heat transfer with a maximum growth of 0.1 wt %. The maximum heat transfer was observed for ZnO + Al2O3+TiO2/DW based Ternary hybrid composite nanofluids at 0.1 wt % concentrations, up to 900–5700 W/m2K, which is 89% higher than distilled water. While, an enhancement of 900–3870 W/m2K, 900–3350 W/m2K, 900–2750 W/m2K were observed for the other three wt. % 0.075, 0.05 and 0.025, respectively. The study revealed that the metal oxide based ternary hybrid composites nanofluids are suitable for nano coolant applications due to improved thermophysical characteristics and also it is applicable for energy management in industrial applications.

Interaction of fusion temperature on the magnetic free convection of nano-encapsulated phase change materials within two rectangular fins-equipped porous enclosure

Abstract: The present study encountered the impact of non-dimensional fusion temperature on the free convection of conducting nanofluid within a porous enclosure filled with nano-encapsulated phase change materials (NEPCMs). The enclosure is equipped with two parallel fins that have ability to move in both directions such as vertically as well as horizontally. In particular the particles are structured as core-shell with phase change materials. The phase change of the materials is obtained from the solid to liquid and absorbs the surrounding temperature in the hot region and releases in the cold region. The governing transformed equations are tackled by using the Finite Element Method (FEM). The numerical simulation of the isotherms, streamlines and heat transfer coefficient ratio along with velocity distribution for various parameters are presented. These are affecting a key role on the average and local Nusselt number as well as on the local Bejan number. However, the measure outcomes are; both the longitudinal and transverse velocity profiles boost up with an augmented Rayleigh number; however, the weaker flow field is generated for the increasing Hartmann number.

Unsteady squeezing flow of Cu-Al2O3/water hybrid nanofluid in a horizontal channel with magnetic field.

Abstract: The proficiency of hybrid nanofluid from Cu-Al2O3/water formation as the heat transfer coolant is numerically analyzed using the powerful and user-friendly interface bvp4c in the Matlab software For that purpose, the Cu-Al2O3/water nanofluid flow between two parallel plates is examined where the lower plate can be deformed while the upper plate moves towards/away from the lower plate Other considerable factors are the wall mass suction/injection and the magnetic field that applied on the lower plate The reduced ordinary (similarity) differential equations are solved using the bvp4c application The validation of this novel model is conducted by comparing a few of numerical values for the reduced case of viscous fluid The results imply the potency of this heat transfer fluid which can enhance the heat transfer performance for both upper and lower plates approximately by 710% and 411%, respectively An increase of squeezing parameter deteriorates the heat transfer coefficient by 428% (upper) and 535% (lower), accordingly The rise of suction strength inflates the heat transfer at the lower plate while the presence of the magnetic field shows a reverse result