H.A. Mohammed, G. Bhaskaran, N.H. Shuaib, H.I. Abu‐Mulaweh and R. Saidur
The purpose of this paper is to investigate numerically the thermal and hydrodynamics performance of circular microchannel heat exchanger (CMCHE) using various nanofluids.
Abstract
Purpose
The purpose of this paper is to investigate numerically the thermal and hydrodynamics performance of circular microchannel heat exchanger (CMCHE) using various nanofluids.
Design/methodology/approach
The three‐dimensional steady, laminar developing flow and conjugate heat transfer governing equations of a balanced MCHE are solved using finite volume method.
Findings
The results are shown in terms of temperature profile, heat transfer coefficient, pressure drop, wall shear stress, pumping power, effectiveness and performance index. The addition of nanoparticles increased the heat transfer rate of the base fluids. The temperature profiles of the fluids have revealed that higher average bulk temperatures were obtained by the nanofluids compared to water. The addition of nanoparticles also increased the pressure drop along the channels slightly. The increase in nanoparticle concentrations yielded better heat transfer rate while the increase in Reynolds number decreased the heat transfer rate.
Research limitations/implications
The tested nanofluids are Ag, Al2O3, CuO, SiO2, and TiO2. Reynolds number range varied from 100 to 800 and the nanoparticle concentration varied from 2 per cent to 10 per cent.
Practical implications
Parallel flow in CMCHEs is used in thermal engineering applications and the design and performance analysis of these CMCHE are of practical importance.
Originality/value
This paper provides the details of the thermal and hydrodynamics performance analysis of flow heat exchangers using nanofluids, which can be used for heat transfer augmentation in thermal design.
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Masoud Darbandi, Mohammad Taeibi‐Rahni and Ali Reza Naderi
One major challenge in turbulent flow applications is to control the recirculation zone behind the backward‐facing step (BFS). One simple idea to do so is to modify the original…
Abstract
Purpose
One major challenge in turbulent flow applications is to control the recirculation zone behind the backward‐facing step (BFS). One simple idea to do so is to modify the original BFS geometry, of course, without causing adverse or undesirable impacts on the original characteristics of the primary stream. The main objective of this work is to examine the solidity of the recirculation zone behind several different geometries which are slightly to moderately different from the original BFS geometry.
Design/methodology/approach
The implemented modifications cause complicated irregularities at the boundaries of the domain. The experience shows that the mesh distribution around these irregularities plays a critical role in the accuracy of the numerical solutions. To achieve the most accurate solutions with the least computational efforts, we use a robust hybrid strategy to distribute the computational grids in the domain. Additionally, a suitable numerical algorithm capable of handling hybrid grid topologies is properly extended to analyze the flow field. The current fully implicit method utilizes a physical pressure‐based upwinding scheme capable of working on hybrid mesh.
Findings
The extended algorithm is very robust and obtains very accurate solutions for the complex flow fields despite utilizing very coarse grid resolutions. Additionally, different proposed geometries revealed very similar separated regions behind the step and performed minor differences in the location of the reattachment points.
Research limitations/implications
The current study is fulfilled two‐dimensionally. However, the measurements in testing regular BFS problems have shown that the separated shear layer behind the step is not affected by 3D influences provided that the width of channel is sufficiently wide. A similar conclusion is anticipated here.
Practical implications
The problem occurs in the pipe and channel expansions, combustion chambers, flow over flying objects with abrupt contraction on their external surfaces, etc.
Originality/value
A novel pressure‐based upwinding strategy is properly employed to solve flow on multiblocked hybrid grid topologies. This strategy takes into account the physics associated with all the transports in the flow field. To study the impact of shape improvement, several modified BFS configurations were suggested and examined. These configurations need only little additional manufacturing cost to be fabricated.
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Nawar Mohammed Ridha Hashim, Mohd. Zamri Yusoff and Hussein Ahmed Mohammed
The purpose of this paper is to numerically study the phenomenon of separation and subsequent reattachment that happens due to a sudden contraction or expansion in flow geometry…
Abstract
Purpose
The purpose of this paper is to numerically study the phenomenon of separation and subsequent reattachment that happens due to a sudden contraction or expansion in flow geometry, in addition, to investigating the effect of nanoparticles suspended in water on heat transfer enhancement and fluid flow characteristics.
Design/methodology/approach
Turbulent forced convection flow over triple forward facing step (FFS) in a duct is numerically studied by using different types of nanofluids. Finite volume method is employed to carry out the numerical investigations. with nanoparticles volume fraction in the range of 1-4 per cent and nanoparticles diameter in the range 30-75 nm, suspended in water. Several parameters were studied, such as the geometrical specification (different step heights), boundary conditions (different Reynolds [Re] numbers), types of fluids (base fluid with different types of nanoparticles), nanoparticle concentration (different volume fractions) and nanoparticle size.
Findings
The numerical results indicate that the Nusselt number increases as the volume fraction increases, but it decreases as the diameter of the nanoparticles of nanofluids increases. The turbulent kinetic energy and its dissipation rate increase as Re number increases. The velocity magnitude increases as the density of nanofluids decreases. No significant effect of increasing the three steps heights on Nusselt along the heated wall, except in front of first step where increasing the first step height leads to an increase in the recirculation zone size adjacent to it.
Research limitations/implications
The phenomenon of separation and subsequent reattachment happened due to a sudden contraction or expansion in flow geometry, such as forward facing and backward facing steps, respectively, can be recognized in many engineering applications where heat transfer enhancement is required. Some examples include cooling systems for electronic equipment, heat exchanger, diffusers and chemical process. Understanding the concept of these devices is very important from the engineering point of view.
Originality/value
Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions, the traditional fluids or by enhancing thermal conductivity of the fluid. Great attention has been paid to increase the thermal conductivity of base fluid by suspending nano-, micro- or larger-sized particles in fluid. The products from suspending these particles in the base fluid are called nanofluids. Many studies have been conducted to investigate the heat transfer and fluid flow characteristics over FFS. This study is the first where nanofluids are employed as working fluids for flow over triple FFS.
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A.B. Ansari and S.A. Gandjalikhan Nassab
The purpose of this paper is to focus on thermal characteristics behavior of forced convection flow in a duct over forward facing step (FFS), in which all of the heat transfer…
Abstract
Purpose
The purpose of this paper is to focus on thermal characteristics behavior of forced convection flow in a duct over forward facing step (FFS), in which all of the heat transfer mechanisms, including convection, conduction and radiation, take place simultaneously in the fluid flow.
Design/methodology/approach
The fluid is treated as a gray, absorbing, emitting and scattering medium. The Navier‐Stokes and energy equations are solved numerically by computational fluid dynamics (CFD) techniques to obtain the velocity and temperature fields. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation heat transfer take place simultaneously in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinate method (DOM) to find the radiative heat flux distribution inside the radiating medium. By this numerical approach, the velocity, pressure and temperature fields are calculated.
Findings
The effect of wall emissivity, optical thickness, albedo coefficient and the radiation‐conduction parameter on heat transfer behavior of the system are also investigated. The numerical results for two cases of convection‐conduction and conduction‐radiation problems are compared with the available data published in open literature and good agreement was obtained.
Originality/value
This is the first time in which flow over FFS in a duct, considering all heat transfer mechanisms including conduction, convection and radiation, is solved numerically.
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Hydrothermal waves represent the preferred mode of instability of the so-called Marangoni flow for a wide range of liquids and conditions. The related features in classical…
Abstract
Purpose
Hydrothermal waves represent the preferred mode of instability of the so-called Marangoni flow for a wide range of liquids and conditions. The related features in classical rectangular containers have attracted much attention over recent years owing to the relevance of these oscillatory modes to several techniques used for the production of single crystals of semiconductor or oxide materials. Control or a proper knowledge of convective instabilities in these systems is an essential topic from a material/product properties saving standpoint. The purpose of this study is to improve our understanding of these phenomena in less ordinary circumstances.
Design/methodology/approach
This short paper reports on a numerical model developed to inquire specifically about the role played by sudden changes in the available cross-section of the shallow cavity hosting the liquid. Although accounting for the spanwise dimension would be necessary to derive quantitative results, the approach is based on the assumption of two-dimensional flow, which, for high-Pr fluids, is believed to retain the essence of the involved physical processes.
Findings
Results are presented for the case of a fluid with Pr = 15 filling an open container with a single backward-facing or forward-facing step on the bottom wall or with an obstruction located in the centre. It is shown that the presence of steps in the considered geometry can lead to a variety of situations with significant changes in the local spectral content of the flow and even flow stabilization in certain circumstances. The role of thermal boundary conditions is assessed by considering separately adiabatic and conducting conditions for the bottom wall.
Originality/value
Although a plethora of studies have been appearing over recent years motivated, completely or in part, by a quest to identify new means to mitigate these instabilities and produce accordingly single crystals of higher quality for the industry, unfortunately, most of these research works were focusing on very simple geometries. In the present paper, the causality and interdependence among all the kinematic and thermal effects mentioned above is discussed.
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Erdem Cuce and Pinar Mert Cuce
The purpose of this paper is to investigate the effects of concavity level on performance parameters of a parabolic fin under the influences of natural convection and radiation…
Abstract
Purpose
The purpose of this paper is to investigate the effects of concavity level on performance parameters of a parabolic fin under the influences of natural convection and radiation.
Design/methodology/approach
Computational fluid dynamics software (FLUENT) is used for the heat transfer analysis. Optimum fin geometry is searched in order to maximize the heat dissipation from fin to the ambient while minimizing the volume of fin.
Findings
The fin profile with concavity level of 2 dissipates 14.92, 17.53, 24.33 and 26.60 percent more heat and uses 34.62, 49.64, 57.66 and 63.09 percent much material compared to the fin with concavity level of 4, 6, 8 and 10, respectively. It is also observed that the amount of heat dissipation per mass considerably increases with increasing concaveness.
Research limitations/implications
The research was carried out for five different concavity levels in the range of 2-10.
Practical implications
The results can be used in passive cooling applications of PV systems. Also, heat sinks for CPU cooling can be redesigned with respect to the results obtained from the research.
Originality/value
In this paper, effects of concavity level on performance parameters of a parabolic fin are investigated for the first time. It is observed from the numerical results that the fin profile with higher concavity levels provides a cheaper and lighter heat dissipation device so it is recommended for the applications where the weight and the cost are primary considerations such as cooling of photovoltaics.
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The purpose of this paper is to numerically investigate the effects of different heating and cooling scenarios on the flow structure in a vertically oriented plane sudden…
Abstract
Purpose
The purpose of this paper is to numerically investigate the effects of different heating and cooling scenarios on the flow structure in a vertically oriented plane sudden expansion. Four different heating and cooling scenarios are considered. The scenarios include symmetric or asymmetric heating or cooling of the downstream channel walls.
Design/methodology/approach
The governing equations are formulated using a stream function‐vorticity approach. Second‐order accurate central differencing is used to discretize all terms, including the nonlinear convection terms in the vorticity transport and energy equations. Numerical test cases are simulated for Reynolds number values up to 200 and Grashof number values up to 400.
Findings
Numerical simulations show that symmetric heating results in the reduction and ultimately the elimination of flow separation zones near the channel walls while creating a region of reversed flow in the core. On the other hand, symmetric cooling causes the flow to adopt a wavy structure which significantly enhances heat transfer due to jet impingement effects. Finally, it is shown that asymmetric heating causes the flow to preferentially attach to the high‐temperature wall while asymmetric cooling causes the flow to separate completely from the low‐temperature wall.
Originality/value
The behaviour of fluid flow in a plane sudden expansion under symmetric heating is available in the literature. In the present study, the flow structure under alternative heating and cooling scenarios is investigated for the first time.
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Xiong Xiang, Yu Fan, Wei Liu and Aiwu Fan
The purpose of this paper is to compare the thermal resistances between optimized gallium- and water-based heat sinks to show which one is superior.
Abstract
Purpose
The purpose of this paper is to compare the thermal resistances between optimized gallium- and water-based heat sinks to show which one is superior.
Design/methodology/approach
Taking the thermal resistances of heat sinks as the goal function, an optimization process is programmed based on the genetic algorithm. The optimal channel/fin widths and the corresponding thermal resistances of gallium- and water-based heat sinks are obtained and compared with/without a laminar flow constraint. The analytic model and CFD method are applied in different situations to ensure sufficient accuracy.
Findings
The results show that in the laminar regime, the thermal resistance of optimized gallium-based heat sink is lower than the water-based counterpart in most cases, but the latter becomes better if it is long enough or the channel is sufficient high. Without the laminar constraint, the thermal resistance of the optimized gallium-based heat sink can be decreased by 33-45 per cent compared with the water-based counterparts. It is interesting to find that when the heat sink is long or the channel height is short, the optimal geometry of gallium-based heat sink is a mini gap.
Originality/value
This paper demonstrates that the cooling performance of gallium-based heat sink can be significantly improved by optimization without the laminar flow constraint.
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Chongli Di, Xiaohua Yang, Xuejun Zhang, Jun He and Ying Mei
The purpose of this paper is to simulate and analyze accurately the multi-scale characteristics, variation periods and trends of the annual streamflow series in the Haihe River…
Abstract
Purpose
The purpose of this paper is to simulate and analyze accurately the multi-scale characteristics, variation periods and trends of the annual streamflow series in the Haihe River Basin (HRB) using the Hilbert-Huang Transform (HHT).
Design/methodology/approach
The Empirical Mode Decomposition (EMD) approach is adopted to decompose the original signal into intrinsic mode functions (IMFs) in multi-scales. The Hilbert spectrum is applied to each IMF component and the localized time-frequency-energy distribution. The monotonic residues obtained by EMD can be treated as the trend of the original sequence.
Findings
The authors apply HHT to 14 hydrological stations in the HRB. The annual streamflow series are decomposed into four IMFs and a residual component, which exhibits the multi-scale characteristics. After the Hilbert transform, the instantaneous frequency, center frequency and mean period of the IMFs are obtained. Common multi-scale periods of the 14 series exist, e.g. 3.3a, 4∼7a, 8∼10a, 11-14a, 24∼25a and 43∼45a. The residues indicate that the annual streamflow series has exhibited a decreasing trend over the past 50 years.
Research limitations/implications
The HHT method is still in its early stages of application in hydrology and needs to be further tested.
Practical implications
It is helpful for the study of the complex features of streamflow.
Social implications
This paper will contribute to the sustainable utilization of water resources.
Originality/value
This study represents the first use of the HHT method to analyze the multi-scale characteristics of the streamflow series in the HRB. This paper provides an important theoretical support for water resources management.
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Hsien-Hung Ting and Shuhn-Shyurng Hou
The purpose of this paper is to numerically investigate the convective heat transfer of water-based CuO nanofluids flowing through a square cross-section duct under constant heat…
Abstract
Purpose
The purpose of this paper is to numerically investigate the convective heat transfer of water-based CuO nanofluids flowing through a square cross-section duct under constant heat flux in the turbulent flow regime.
Design/methodology/approach
The numerical simulation is carried out at various Peclet numbers and particle concentrations (0.1, 0.2, 0.5, and 0.8 vol%). The finite volume formulation is used with the semi-implicit method for pressure-linked equations algorithm to solve the discretized equations derived from the partial nonlinear differential equations of the mathematical model.
Findings
The heat transfer coefficients and Nusselt numbers of CuO-water nanofluids increase with increases in the Peclet number as well as particle volume concentration. Also, enhancement of the heat transfer coefficient is much greater than that of the effective thermal conductivity at the same nanoparticle concentration.
Research limitations/implications
Simulation of nanofluids turbulent forced convection at very high Reynolds number is worth for further study.
Practical implications
The heat transfer rates through non-circular ducts are smaller than the circular tubes. Nevertheless, the pressure drop of the non-circular duct is less than that of the circular tube. This study clearly presents that the nanoparticles suspended in water enhance the convective heat transfer coefficient, despite low volume fraction between 0.1 and 0.8 percent. Adding nanoparticles to conventional fluids may enhance heat transfer performance through the non-circular ducts, leading to extensive practical applications in industries for the non-circular ducts.
Originality/value
Few papers have numerically studied convective heat transfer properties of nanofluids through non-circular ducts. The present numerical results show a good agreement with the published experimental data.