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Showing papers in "Journal of Thermal Science and Engineering Applications in 2015"



Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the effect of the convergent angle and cold orifice diameter on the thermal efficiency of a convergent vortex tube (CVT), which is linked to an air pipeline with the fixed pressure of 6.5 bar.
Abstract: The vortex tube (VT) air separator is an invaluable tool which has the ability to separate a high-pressure fluid into the cold and hot fluid streams. The hot tube is a main part of the air separator VT which the energy separation procedure happens along this part. This research has been done to analyze the effect of the convergent angle and cold orifice diameter on the thermal efficiency of a convergent vortex tube (CVT). The CVT is linked to an air pipeline with the fixed pressure of 6.5 bar. The convergent hot tube angle is varied over the range of 1 deg to 9 deg. The consideration of the main angle effect denotes that the highest thermal ability could be achieved at β = 5 deg. The laboratory setup results show this subject that the optimization of the hot tube convergent angle elevates the cooling and heating effectiveness around 32.03% and 26.21%, respectively. Experiments denoted that both cooling capability and heating effectiveness reach the highest magnitudes when the DCold is around 9 mm. After these two stages, the optimized CVT was capable of decreasing and rising air temperatures at the cold and the hot sides up to 9.05 K (42.89%) and 10.48 K (44.74%), respectively. A computational fluid dynamics (CFD) model was employed to predict the performance of the air flow inside the CVT. The numerical investigation was done by full 3D steady-state CFD-simulation using fluent6.3.26. The results show that the agreement between computation predictions and laboratory measurements is fairly good.

35 citations






Journal ArticleDOI
TL;DR: In this paper, a two-dimensional unsteady state numerical model was developed for simulation of the heat and mass transfer phenomena in a representative channel of a dehumidification desiccant wheel (DW) matrix.
Abstract: This paper focused on the exergy analysis and optimization of a dehumidification desiccant wheel (DW) system. A two-dimensional unsteady state numerical model was developed for simulation of the heat and mass transfer phenomena in a representative channel of a DW matrix. The DW mathematical model was validated using a series of experimental data and parametric studies were conducted to investigate the effects of operating parameters on the DW system performance. Exergy parameters were also studied and adopted to predict the total inlet–outlet exergy and exergy destruction, as well as exergy effectivenesses. Furthermore, a new exergy effectiveness parameter was introduced based on the concept of dehumidification. Parametric studies were carried out to characterize the optimal performance of the overall system regarding exergy destruction and exergy dehumidification effectivenesses. The results demonstrate that electrical power consumption, regeneration heat, and heat and mass transfer between air and desiccant are the main sources of exergy destruction. The optimization calculation shows that at the lowest process air velocity (up = 0.2 m/s), lowest DW rotational speed (NDW = 4 Rph), highest regeneration air temperature (Ta,r,in = 140 °C), and moderate regeneration air velocity (ur = 1.7 m/s), minimum exergy destruction occurs. The optimal value of the parameters proves that, when exergy destruction effectiveness is selected as the objective function, the only regeneration air velocity is decision variable of optimization and operational limits impose on the other parameters.

26 citations


Journal ArticleDOI
TL;DR: In this article, the average Nusselt numbers of the two sides of the heat exchangers and the overall heat transfer coefficients increased by increasing coil curvature ratio, and a significant increase of 33.2-7.7% was obtained in the HCT-Fanning friction factor when δ increases from 0.0392 to 0.1194 within the investigated ranges of different parameters.
Abstract: The present work experimentally investigates the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to friction factor for fully developed flow through the helically coiled tube (HCT). The majority of previous studies were performed on HCTs with isothermal and isoflux boundary conditions or shell and coil heat exchangers with small ranges of HCT configurations and fluid operating conditions. Here, five heat exchangers of counter-flow configuration were constructed with different HCT-curvature ratios (δ) and tested at different mass flow rates and inlet temperatures of the two sides of the heat exchangers. Totally, 295 test runs were performed from which the HCT-side and shell-side heat transfer coefficients were calculated. Results showed that the average Nusselt numbers of the two sides of the heat exchangers and the overall heat transfer coefficients increased by increasing coil curvature ratio. The average increase in the average Nusselt number is of 160.3–80.6% for the HCT side and of 224.3–92.6% for the shell side when δ increases from 0.0392 to 0.1194 within the investigated ranges of different parameters. Also, for the same flow rate in both heat exchanger sides, the effect of coil pitch and number of turns with the same coil torsion and tube length is remarkable on shell average Nusselt number while it is insignificant on HCT-average Nusselt number. In addition, a significant increase of 33.2–7.7% is obtained in the HCT-Fanning friction factor (fc) when δ increases from 0.0392 to 0.1194. Correlations for the average Nusselt numbers for both heat exchanger sides and the HCT Fanning friction factor as a function of the investigated parameters are obtained.

24 citations


Journal ArticleDOI
TL;DR: In this article, a radial flat-plate oscillating heat pipe (RFP-OHP) heat spreader is investigated to study the effect of central heating on the heat transport capability in an OHP.
Abstract: With ever increasing technological advances in electronics, modern computer components continue to produce higher power densities that present a challenge to thermal management. A radial flat-plate oscillating heat pipe (RFP-OHP) heat spreader is investigated to study the effect of central heating on the heat transport capability in an OHP. The investigated OHP has dimensions of 100 mm × 100 mm × 2.5 mm with central heating using a 30 mm × 30 mm heater. Experimental results show that when heat is added to the center section of one side of the radial flat-plate OHP, and when heat is removed from the whole surface of another side of the heat pipe, the startup power for the oscillating motion increases. In addition, the spacer effect on the heat transport capability including the startup is investigated experimentally. The spacer added between the cooling block and OHP could lower the startup power for oscillatory motion. When compared to a copper slab of the same dimensions in the same test configuration, the temperature difference for the OHP with and without the additional copper spacer was reduced by a maximum of 46% and 25%, respectively, at a power input of 525 W and a heat flux of 58 W/cm2.

22 citations


Journal ArticleDOI
TL;DR: In this article, the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to the friction factor for fully developed flow through their helically coiled tube (HCT) were investigated.
Abstract: This study presents an experimental investigation of the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to the friction factor for fully developed flow through their helically coiled tube (HCT). Five heat exchangers of counterflow configuration were constructed with different HCT-curvature ratios (δ) and tested at different mass flow rates and inlet temperatures of γ-Al2O3/water nanofluid in the HCT. The tests were performed for γ-Al2O3 with average size of 40 nm and particles volume concentration (ϕ) from 0% to 2% for 0.0392≤δ≤0.1194. Totally, 750 test runs were performed from which the HCT-average Nusselt number (Nu¯t) and fanning friction factor (fc) were calculated. Results illustrated that Nu¯t and fc of nanofluids are higher than those of the pure water at same flow condition, and this increase goes up with the increase in ϕ. When ϕ increases from 0% to 2%, the average increase in Nu¯t is of 59.4–81% at lower and higher HCT-Reynolds number, respectively, and the average increase in fc is of 25.7% and 27.4% at lower and higher HCT-Reynolds number, respectively, when ϕ increases from 0% to 2% for δ=0.1194. In addition, results showed that Nu¯t and fc increase by increasing coil curvature ratio. When δ increases from 0.0392 to 0.1194 for ϕ=2%, the average increase in Nu¯t is of 130.2% and 87.2% at lower and higher HCT-Reynolds number, respectively, and a significant increase of 18.2–7.5% is obtained in the HCT-fanning friction factor at lower and higher HCT-Reynolds number, respectively. Correlations for Nu¯t and fc as a function of the investigated parameters are obtained.

20 citations



Journal ArticleDOI
TL;DR: In this paper, the performance of axial flow fans in air-cooled steam condenser systems was investigated under 2D wind conditions, which are characterized by the separation on the upstream edge of the fan inlets, and a correlation between system volumetric e ectiveness, platform height and cross-wind velocity was found.
Abstract: Simulating the e ect of wind on the performance of axial ow fans in air-cooled steam condenser systems N. Fourie Department of Mechanical and Mechatronic Engineering University of Stellenbosch Private Bag X1, 7602 Matieland, South Africa Thesis: MEng (Mech) December 2014 The use of air-cooled steam condensers (ACSCs) is the preferred cooling method in the chemical and power industry due to stringent environmental and water use regulations. The performance of ACSCs is however highly dependent on the in uence of windy conditions. Research has shown that the presence of wind reduces the performance of ACSCs. It has been found that cross-winds (wind perpendicular to the longest side of the ACSC) cause distorted inlet ow conditions, particularly at the upstream peripheral fans near the symmetry plane of the ACSC. These fans are subjected to what is referred to as `2-D' wind conditions, which are characterised by ow separation on the upstream edge of the fan inlets. Experimental investigations into inlet ow distortion have simulated these conditions by varying the fan platform height. Low platform heights resulted in higher levels of inlet ow distortion, as also found to exist with high cross-wind speeds. This investigation determines the performance of various fan con gurations (representative of con gurations used in the SouthAfrican power industry) subjected to distorted inlet ow conditions through experimental and numerical investigations. The similarity between platform height and cross-wind e ects is also investigated and a correlation between system volumetric e ectiveness, platform height and cross-wind velocity is found. ii Stellenbosch University http://scholar.sun.ac.za

Journal ArticleDOI
TL;DR: In this paper, a well-established resistance-capacitance (RC) model is employed that utilizes a representative network of electric resistors and capacitors to simulate the thermal behavior of such systems.
Abstract: Simulating the real-time thermal behavior of rooms subject to air conditioning and refrigeration is a key to cooling load c alculations. A well-established Resistance-Capacitance (RC) model is employed that utilizes a representative network of electric resistors and capacitors to simulate the thermal behavior of such systems. A freezer room of a restaurant is studied during its operation and temperature measurements are used for model validation. Parametric study is performed on different properties of the system. It is shown that a reduction of 20% in the walls thermal resistivity can increase the energy consumption rate by 15%. The effect of set points on the number of compressor starts/stops is also studied and it is shown that narrow set points can result in a steady temperature pattern in exchange for a high number of compressor starts/stops per hour. The proposed technique provides an effective tool for facilitating the thermal modeling of air conditioned and refrigerated rooms. Using this approach, engineering calculations of cooling load can be performed with outstanding simplicity and accuracy.

Journal ArticleDOI
TL;DR: In this article, an explicit transient simulation is performed to solve the governing equations including the source terms for heat and mass transfer due to condensation, and a piecewise linear interface calculation (PLIC) method is employed to keep the interface sharp.
Abstract: The understanding of multiple bubbles condensation is of significant importance in developing continuum models for the large-scale subcooled flow boiling. The computational fluid dynamics (CFD) modeling for multiple bubbles condensation is developed with the volume of fluid (VOF) method in this work. An explicit transient simulation is performed to solve the governing equations including the source terms for heat and mass transfer due to condensation. The geometric reconstruction scheme, which is a piecewise linear interface calculation (PLIC) method, is employed to keep the interface sharp. The surface tension is modeled by the continuum surface force (CSF) approach, which is taken into account in the numerical model. Numerical simulations predict the dynamical behavior of the actual condensing bubbles. The results show that the condensation rate of a single bubble is influenced by the velocity of the fluid flow and the temperature difference between the bubble and fluid. For multiple bubbles, the effect of bubble-bubble interaction on their condensation process is analyzed based on the numerical predictions. The condensation rate of lower bubbles increases due to the random perturbation induced by other bubbles. The influence of other bubbles on the condensation rate can be neglected if the distances between the bubbles are large enough. (Less)

Journal ArticleDOI
TL;DR: In this article, the performance of a pin-finned heat sink was investigated by using two widely used nanofluids, Al2O3-water and TiO2-water.
Abstract: In this paper, the hydrodynamic and thermal performance of a miniature plate pin-finned heat sink is investigated experimentally by utilizing two widely used nanofluids, Al2O3– water and TiO2–water. The heat sink base plate, which is used in the cooling process of electronic devices, has the dimensions of 42 mm (L) 42 mm (W) 14 mm (H) and is made of aluminum and placed in a plexiglass case which is isolated from the environment using an insulator foam. The thermal performance of the heat sink is investigated by passing the nanofluid at constant inlet temperature while applying a constant heat flux of 124.8 kW/m to the bottom surface of the heat sink. The nanofluids are prepared in volume concentrations of 0.5, 1, 1.5, and 2% and their performances are measured considering water as the base fluid. Measuring the pressure difference between the entrance and exit of the heat sink made it possible to study the hydrodynamic performance of the heat sink. Although the measurements showed 15% and 30% increase in the pumping power for the volume concentration of 2% of Al2O3–water and TiO2–water nanofluids, respectively, the average heat transfer coefficients increased by 16% and 14% and the thermal resistance decreased by 17% and 14% for each nanofluid. [DOI: 10.1115/1.4030103]

Journal ArticleDOI
TL;DR: In this article, a low cost solar-thermoelectric (TE) air-conditioning system for people in remote areas where electricity is still in short supply was designed and constructed.
Abstract: In this paper, we have designed and constructed a low cost solar-thermoelectric (TE) air-conditioning system for people in remote areas where electricity is still in short supply. Such system can be potentially used to condition tents and living areas. The proposed solar-powered TE air-conditioning system is based on the principles of Peltier effect to create a finite temperature difference across the condenser and the evaporator of the TE air-conditioning system. The cold side (or the evaporator) of the TE module is used for air-conditioning application; provides cooling to the living space. The thermal energy from the hot side of the module is dumped to the surrounding environment. Using the existing heat transfer and thermodynamics knowledge, an analytical model is developed to predict the performance of the solar-TE air-conditioning system in terms of the hot and cold reservoir temperatures, heat removal rates from the conditioned space, power input, and coefficient of performance (COP). A second analytical model is proposed to predict the cooling down period of the conditioned space as a function of heat removed by air-conditioning system, heat gained through the wall of the conditioned space, and heat generated inside the conditioned space. A detailed system is constructed to predict the performance of solar-TE air-conditioning system experimentally. A conditioned space was constructed to carry out the experimental work. Multiple air-conditioning systems were installed in the conditioned space. The cooling performance of the designed solar-TE air-conditioning system was experimentally tested and verified with the analytical calculation.



Journal ArticleDOI
TL;DR: In this paper, the effect of rotation on heat transfer in a turbine blade serpentine coolant passage with ribbed walls at low Mach numbers was investigated, and the results showed an increase in heat transfer rates due to rotation.
Abstract: This paper experimentally investigates the effect of rotation on heat transfer in typical turbine blade serpentine coolant passage with ribbed walls at low Mach numbers. To achieve the low Mach number (around 0.01) condition, pressurized Freon R-134a vapor is utilized as the working fluid. The flow in the first passage is radial outward, after the 180 deg tip turn the flow is radial inward to the second passage, and after the 180 deg hub turn the flow is radial outward to the third passage. The effects of rotation on the heat transfer coefficients were investigated at rotation numbers up to 0.6 and Reynolds numbers from 30,000 to 70,000. Heat transfer coefficients were measured using the thermocouples-copper-plate-heater regional average method. Heat transfer results are obtained over a wide range of Reynolds numbers and rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Regional heat transfer coefficients are correlated with Reynolds numbers for nonrotation and with rotation numbers for rotating condition, respectively. The results can be useful for understanding real rotor blade coolant passage heat transfer under low Mach number, medium–high Reynolds number, and high rotation number conditions.

Journal ArticleDOI
TL;DR: In this article, a DoE simulation of 396 cases focusing on the arrangement of the combined-hole with double holes for improving film cooling performance is carried out in this work, and the influence of an aerodynamic parameter, blowing ratio is considered as well.
Abstract: Film cooling technique is widely used to protect the components from being destroyed by hot mainstream in a modern gas turbine. Combining round-holes is a promising way of improving film cooling effectiveness. A DoE (design of experiment) simulation of 396 cases focusing on the arrangement of the combined-hole with double holes for improving film cooling performance is carried out in this work, and the influence of an aerodynamic parameter, blowing ratio is considered as well. The dimensionless lateral distance (PoD) and compound angle (CA) of the double holes have relative influence on the film cooling performance of the combined-hole unit. At the low blowing ratio, increasing symmetrical compound angle (SCA) has positive influence on the area-average effectiveness (EFF) of the combined-hole. But at the intermediate and large blowing ratio, the influence of SCA on the area-average EFF depends on the range of PoD. At the small PoD, the area-average EFF ascends basically along SCA axis. However, the area-average EFF first ascends and subsequently descends along SCA axis at the large PoD. Asymmetrical compound angle (ACA) is also considered to fit the antikidney vortexes produced in the combined-hole film cooling compared to their ideal schematic. However, the film cooling effect of the cases with ACA is not as good as expected. The area-average EFF of ACA cases locates in the level between that of the adjacent SCA cases. The optimal arrangement of combined-hole unit for improving film cooling effectiveness is relative to the local flow field. The optimal arrangement of PoD and CA for improving the combined-hole film cooling performance is different at different blowing ratios.

Journal ArticleDOI
TL;DR: In this article, a dynamic thermal model for a cross flow heat exchanger is solved numerically in order to predict the transient response under step changes in the fluid mass flow rate and the fluid inlet temperature.
Abstract: Heat exchangers are important facilities that are widely used in heating, ventilating, and air conditioning (HVAC) systems. For example, heat exchangers are the primary units used in the design of the heat transfer loops of cooling systems for data centers. The performance of a heat exchanger strongly influences the thermal performance of the entire cooling system. The prediction of transient phenomenon of heat exchangers is of increasing interest in many application areas. In this work, a dynamic thermal model for a cross flow heat exchanger is solved numerically in order to predict the transient response under step changes in the fluid mass flow rate and the fluid inlet temperature. Transient responses of both the primary and secondary fluid outlet temperatures are characterized under different scenarios, including fluid mass flow rate change and a combination of changes in the fluid inlet temperature and the mass flow rate. In the e-NTU (number of transfer units) method, the minimum capacity, denoted by Cmin, is the smaller of Ch and Cc. Due to a mass flow rate change, Cmin may vary from one fluid to another fluid. The numerical procedure and transient response regarding the case of varying Cmin are investigated in detail in this study. A review and comparison of several journal articles related to the similar topic are performed. Several sets of data available in the literatures which are in error are studied and analyzed in detail.





Journal ArticleDOI
TL;DR: In this paper, thermal analysis and optimization of the material composition in functionally graded (FG) cutting tools were carried out to achieve the minimum thermal stress by using a generalized coupled thermoelasticity theory of Lord-Shulman based on second sound effect.
Abstract: In this study, thermal analysis and optimization of the material composition in functionally graded (FG) cutting tools were carried out to achieve the minimum thermal stress Since cutting tool particularly rotating ones in milling process are exposed to thermal shock during machining, a complicated analysis is required to analyze the thermal shock response Therefore, a generalized coupled thermoelasticity theory of Lord–Shulman based on second sound effect is adopted Lord–Shulman theory, as a generalized coupled thermoelasticity, is chosen as governing equation in terms of temperature and displacement The coupled equations are transferred to Laplace domain and then Galerkin finite element method is employed to solve the equation in the Laplace domain Then, a numerical Laplace inversion has been applied to transform back the equation from Laplace domain to real time Results are obtained for several material compositions so that the proper composition will be found for design It is shown that FG materials (FGMs) exhibit lower stresses, lower displacement, and lower temperature levels compared to multilayer materials Furthermore, the effect of FGM is increased by increasing the power law index, representing the change in concentration


Journal ArticleDOI
TL;DR: In this paper, the experimental friction factor and Nusselt number data for laminar flow of viscous oil through a circular duct having integral transverse rib roughness and fitted with twisted tapes with oblique teeth are presented.
Abstract: The experimental friction factor and Nusselt number data for laminar flow of viscous oil through a circular duct having integral transverse rib roughness and fitted with twisted tapes with oblique teeth are presented. Predictive friction factor and Nusselt number correlations have been developed. The thermohydraulic performance has been evaluated. The major findings of this experimental investigation are that the twisted tapes with oblique teeth in combination with integral transverse rib roughness perform significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain value of fin parameter.


Journal ArticleDOI
TL;DR: In this paper, numerical simulation and parametric analysis of the geometrical parameters (such as fin density and height) and system parameters are conducted to better understand the performance of the self-cooling system within wide ranges.
Abstract: A heat-driven self-cooling system could potentially utilize the heat dissipated from a device to power a thermo-electric generator (TEG) which could then provide power to run a cooling system. In this paper, numerical simulation and parametric analysis of the geometrical parameters (such as fin density and height) and system parameters are conducted to better understand the performance of the self-cooling system within wide ranges. The study showed further decrease in device temperature could be achieved by using shunt operation instead of direct contact between the device and the TEG module. The use of TEG cascades could also help improve the decrease in power generation as a result of shunt arrangement.