Fluid Temperature Gradient Research Articles

Effect of geometric configuration on the laminar flow and heat transfer in microchannel heat sinks with cavities and fins

ABSTRACTA numerical simulation is performed to investigate the characteristics of flow and heat transfer in microchannels with cavities and fins. Nine microchannels with various shaped cavities and fins are presented and compared to the smooth microchannel. The effect of cavity and fin shapes on the flow field and temperature field is analyzed. Results show that the presence of cavity and fin can increase the heat transfer area, intensify mainstream disturbance, and induce chaotic advection, which result in obvious heat transfer enhancement. The shape of cavity or fin has a great influence on the hydrodynamic and thermal performance for such micro heat sinks. Based on the performance evaluation criterion (PEC), the overall performance of the microchannel is evaluated. The combination of cavities and fins leads to lower bottom temperature, lower net temperature gradient of fluid, and better heat transfer performance, which has the potential to meet the increased heat removal requirement.

Entropy Generation of Nanofluid in a Porous Cavity by Lattice Boltzmann Method

This investigation compared with other numerical studies and found to be in excellent agreement. The results depict the maximum total entropy generation due to the fluid friction and temperature gradient is generated in the vicinity of a high-temperature wall. Also, the results indicate that the average Nusselt number and entropy generation are an increasing function of the Rayleigh number, porosity, and Darcy number, but the entropy generation is a decreasing function of the volume fractions of the nanofluid.

T-junction cross-flow mixing with thermally driven density stratification

As a means of further elucidating turbulence- and stratification-driven thermal fatigue in the vicinity of T-junctions in nuclear power plants, a series of experiments have been conducted at the high temperature high pressure fluid–structure interaction T-junction facility of the University of Stuttgart with novel fluid measurement instrumentation. T-junction mixing with large fluid temperature gradients results in complex flow behavior, the result of density driven effects. Deionized water mixing at temperature differences of up to 232K at 7MPa pressure have been investigated in a T-junction with main pipe diameter 71.8mm and branch line diameter 38.9mm. The experiments have been performed with fixed flow rates of 0.4kg/s in the main pipe and 0.1kg/s in the branch line. A novel electrode-mesh sensor compatible with the DN80 PN100 pipeline upstream and downstream of the T-junction has been utilized as a temperature sensor providing a high density information in the pipe cross-section in both space and time. Additionally, in-flow and in-wall thermocouples quantify the damping of thermal fluctuations by the wall material. The results indicate that large inflow temperature differences lead to strong turbulence damping, and ultimately stable stratification extending both downstream and upstream of the T-junction resulting in large local thermal gradients.

Upper-Convected Maxwell Fluid Flow with Variable Thermo-Physical Properties over a Melting Surface Situated in Hot Environment Subject to Thermal Stratification

An upper-convected Maxwell (UCM) fluid flow over a melting surface situated in hot environment is studied. The influence of melting heat transfer and thermal stratification are properly accounted for by modifying the classical boundary condition of temperature to account for both. It is assumed that the ratio of inertia forces to viscous forces is high enough for boundary layer approximation to be valid. The corresponding influence of exponentially space dependent internal heat generation on viscosity and thermal conductivity of UCM is properly considered. The dynamic viscosity and thermal conductivity of UCM are temperature dependent. Classical temperature dependent viscosity and thermal conductivity models are modified to suit the case of both melting heat transfer and ther- mal stratification. The governing non-linear partial differential equations describing the problem are reduced to a system of nonlinear ordinary differential equations using similarity transformations and completed the solution numerically using the Runge-Kutta method along with shooting technique (RK4SM). The numerical procedure is validated by comparing the solutions of RK4SM with that of MATLAB based bvp4c. The results reveal that increase in stratification parameter corresponds to decrease in the heat energy entering into the fluid domain from freestream and this significantly reduces the overall temperature and temperature gradient of UCM fluid as it flows over a melting surface. The transverse velocity, longitudinal velocity and temperature of UCM are increasing func- tion of temperature dependent viscous and thermal conductivity parameters. At a constant value of melting parameter, the local skin-friction coefficient and heat transfer rate increases with an increase in Deborah number.

Exact Solution of Electroviscous Flow and Heat Transfer in a Semi-annular Microcapillary

The electro-osmotic flow (EOF) and associated heat transfer are investigated in a semi-annular microcapillary. The potential, velocity, and temperature fields are solved by analytic approaches including the eigenfunction expansion and the Green’s function methods. By selecting the potential sign of each surface of the channel, the bulk fluid may flow in two opposite directions. Effects of the key parameters governing the problem are examined. The mass flow rate increases when the hydraulic diameter is increased or the electrokinetic radius is decreased. The results reveal that surface cooling and/or surface heating (of the inner or outer walls) strongly affects the fluid temperature distributions as well as the position of the maximum/minimum temperature region inside the domain; the latter indicates temperature gradients in fluid. Also, higher thermal scale ratio leads to broaden the temperature distribution. Depending on the value of the geometric radius ratio (and for all values of the thermal scale ratio), the fully developed Nusselt number approaches a specific value as the electrokinetic radius tends to infinity.

An investigation of the thermo-hydraulic performance of the smooth wavy fin-and-elliptical tube heat exchangers utilizing new type vortex generators

A three-dimensional CFD numerical simulation is successfully carried out on thermo-hydraulic characteristics of a new smooth wavy fin-and-elliptical tube (SWFET) heat exchanger with three new types of vortex generators (VGs), namely – rectangular trapezoidal winglet (RTW), angle rectangular winglet (ARW) and curved angle rectangular winglet (CARW). Several parameters have been examined in this study. There have a pronounced effect on the thermo-hydraulic performance. In addition the results are analyzed from the viewpoint of the field synergy principle which emphasizes that the reduction of the synergy angle between velocity and fluid temperature gradient is the principal mechanism for enhancement of heat transfer performance. These parameters include: Reynolds number (based on the hydraulic diameter, ReDh=500–3000), geometric shape of VGs, attack angle of VGs (αVG=15–75°), placement of VG pairs (up- or/and downstream), tube ellipticity ratio (e=0.65–1.0) and wavy fin height (H=0.8–1.6mm). The results demonstrate that with increasing Reynolds number and wavy fin height, decreasing the tube ellipticity ratio, the heat transfer performance of the SWFET heat exchanger is enhanced. The SWFET heat exchanger with the advantages of using CARW VGs and RTW VGs at smaller and larger attack angles, respectively, presents good thermo-hydraulic performance enhancement. Finally, new correlations are proposed to estimate the values of the average Nusselt number Nu, friction factor f and synergy angle θ based on the Reynolds number, attack angle of VGs, tube ellipticity ratio and wavy fin height.

Analysis of convective heat transfer improved impeller stirred tanks by the lattice Boltzmann method

This paper performs the lattice Boltzmann simulation combined with unknown-index algorithm for the convective heat transfer problems of an impeller stirred tank. Three types of motion of the heating impeller are adopted in this study, including rotation with constant angular velocity, oscillation and oscillating rotation, which are denoted as Types I, II, and III stirrings respectively. Because the flow structures in rotation stirring case are concentric vertical, the heat transfer efficiency of the tank walls is lower than those of the other two types of stirrings. The other two types of stirrings, the flow structures include complex vortices and present better field synergy of the fluid velocity and temperature gradient, which positively contributes to the convective heat transfer efficiency. The results also indicate that a large swing process angle of the impeller with oscillating rotation stirring does not have significant advantage, because the flow phenomena would be similar to that of constant angular velocity stirring case. Basically, the Type III stirred tank can be considered as an improved Type II stirred tank by choosing appropriate process angle.

Influence of fluid temperature gradient on the flow within the shaft gap of a PLR pump

In nuclear power plants the primary-loop recirculation (PLR) pump circulates the high temperature/high-pressure coolant in order to remove the thermal energy generated within the reactor. The pump is sealed using the cold purge flow in the shaft seal gap between the rotating shaft and stationary casing, where different forms of Taylor–Couette flow instabilities develop. Due to the temperature difference between the hot recirculating water and the cold purge water (of order of 200 °C), the flow instabilities in the gap cause temperature fluctuations, which can lead to shaft or casing thermal fatigue cracks. The present work numerically investigated the influence of temperature difference and rotating speed on the structure and dynamics of the Taylor–Couette flow instabilities. The CFD solver used in this study was extensively validated against the experimental data published in the open literature. Influence of temperature difference on the fluid dynamics of Taylor vortices was investigated in this study. With large temperature difference, the structure of the Taylor vortices is greatly stretched at the interface region between the annulus gap and the lower recirculating cavity. Higher temperature difference and rotating speed induce lower fluctuating frequency and smaller circumferential wave number of Taylor vortices. However, the azimuthal wave speed remains unchanged with all the cases tested. The predicted axial location of the maximum temperature fluctuation on the shaft is in a good agreement with the experimental data, identifying the region potentially affected by the thermal fatigue. The physical understandings of such flow instabilities presented in this paper would be useful for future PLR pump design optimization.

Exergy analysis and performance evaluation of flow and heat transfer in different micro heat sinks with complex structure

Simulations are conducted to investigate flow and heat transfer performance of de-ionized water flowing through different micro heat sinks with complex structure under uniform heat flux. Six new micro heat sinks with different types of cavities and ribs are proposed. Moreover, they are compared involving friction factor and Nusselt number relative to other micro heat sinks in the existed literature. Exergy analysis is also applied to investigate the physical mechanism of heat transfer enhancement. Different performance evaluation criteria, based on first and second law of thermodynamics, are proposed to assess the overall performance of micro heat sinks. The results clearly show that the micro heat sink with triangular cavities and triangular ribs (Tri.C–Tri.R for short) presents the best heat transfer performance over a Reynolds number range of 300–600. The performance of heat transfer can be enhanced by decreasing the net temperature gradient of fluid in micro heat sinks. New correlations of friction factor and Nusselt number are proposed based on numerical data, which relate with Reynolds number and relative cavity height and relative ribs height.

Simulation studies on the transport properties of Cu-H2O nanofluids based on water continuum assumption

Nanofluid is a kind of new engineering medium which is created by dispersing small quantity of nano-sized particles in the base fluid. The dispersion of solid nanoparticles in conventional fluids changes their transport properties remarkably. Molecular dynamics simulation (MDS) is an important approach to study the transport properties of nanofluids. However, the computation amount is huge, and it is very difficult to use the normal MDS to capture the transient flow and heat processes in Cu-H2O nanofluids if the regions in the simulation reach 149.6443 nm3 or 299.2883 nm3, and the number of Cu nano-particles reaches 6-64. Further study by means of simulation on the effects on effective transport properties of nanofluids is also difficult. In this paper, the water-based fluid region of 149.6443 nm3 or 299.2883 nm3 is assumed as continuum phase because of the very low Knudsen number of fluid, and the effects of water on nano-particles are fitted into the Cu-Cu potential parameters. Using the proposed method, the computation amount is significantly reduced. The effective thermal conductivity and dynamic viscosity coefficient of Cu-H2O nanofluids under the stationary condition are simulated and the results are verified with existing experimental data. The motion and aggregation processes of nano-particles in the water-based fluids at different velocity shear rate are simulated. Effects of velocity shear rate, fluid velocity, temperature gradient, and average temperature on the effective thermal conductivity and the dynamic viscosity of Cu-H2O nanofluids in the processes of flow and heat transfer are studied. Three conclusions can be drawn from the obtained results. Firstly, the proposed method is feasible to capture the transient flow and heat processes in Cu-H2O nanofluids, and is also capable to further study the transport properties of Cu-H2O nanofluids. Secondly, the velocity shear rate acting on a nanofluid can effectively prevent the aggregating process of nano-particles, and therefore reduce the diameter of particle-aggregations. Finally, the velocity shear rate and the average temperature of Cu-H2O nanofluids have much more effects on the transport properties, while the fluid velocity and temperature gradient have less effects; the velocity shear rate increases the effective thermal conductivity of a nanofluid but decreases its dynamic viscosity. A rise of average temperature increases the effective thermal conductivity but decreases the dynamic viscosity.

Analysis of zeotropic mixtures used in high-temperature Organic Rankine cycle

The paper investigates the performance of high-temperature Organic Rankine cycle (ORC) with zeotropic mixtures as working fluid. A numerical model, which has been validated by comparing with the published data, is developed to predict the first law thermal efficiency of the cycle. The effects of mixture concentration, temperature gradient of the heat transfer fluid, pinch temperature difference, pressure ratio, and condensation pressure on the first law efficiency are presented firstly using a purposely designed program, and then the suitable conditions for the described ORC are suggested based on the results of the simulation. It is demonstrated that the use of zeotropic mixtures leads to an efficiency increase compared to pure fluids.

Direct numerical simulation of heat and mass transfer of spheres in a fluidized bed

We have developed a direct numerical simulation approach combined with the immersed boundary (DNS–IB) method for studying heat transfer in particulate flows. In this method, fluid velocity and temperature fields are obtained by solving the modified momentum and heat transfer equations, which are due to the presence of heated particles in the fluid; particles are tracked individually and their velocities and positions are solved based on the equations of linear and angular motions; particle temperature is assumed to be constant. The momentum and heat exchanges between a particle and the surrounding fluid at its surface are resolved using the immersed boundary method with the direct forcing scheme. The DNS–IB method has been used to study the heat transfer of 225 heated spheres in a fluidized bed. By exploring the rich data generated from the DNS–IB simulations, we are able to obtain statistically averaged fluid and particle velocities as well as the overall heat transfer rate in the fluidized bed. Good agreement between the current study and the one by Pan et al. (2002) is found for the hydrodynamic properties of the bed such as pressure gradients within the bed and the relationship between fluidization velocity and bed solid fraction. The particle-averaged Nusselt number is found to increase as the fluidization velocity increases and the bed height rises; particles at the entrance of the bed tend to have the maximum heat transfer rate because of the higher particle–fluid temperature gradients in this region; as the fluid moves upward in the bed, it gets warmer, which reduces particle–fluid temperature gradients and decreases the transfer rate of particles.

Structure of Swirl Chamber of Turbulent Combustion Diesel Engine

Numerical simulation was conducted on three types of swirl chamber of turbulent combustion diesel engine, i.e. cone-shaped flat-bottomed, cylindrical flat-bottomed and spherical shape. The characteristics of flow field in cylinder were studied within the time range for the piston to move from BTDC 108°CA to BTDC 8°CA (at the instant of fuel injection), thus analyzed the changes of flow field in swirl chambers of such three different structures prior to fuel injection based on the velocity vector diagram at all times and the final temperature characteristic diagram of the flow field. The results show that: in the process of piston motion, an organized fierce vortex can be developed inside the swirl chamber, while in the vicinity of fuel nozzle, the air flow rate is 111.14m/s, 83.01m/s or 175.76m/s and the air temperature is 1384.15K, 1337.38K or 1350.46K respectively. A small fluid stagnation zone will be formed in the lower right end of the cone-shaped flat-bottomed swirl chamber or the cylindrical flat-bottomed swirl chamber and is adverse to the mixing of fuel and air. In comparison with the swirl chambers of other two structures, the smaller temperature gradient of fluid and the larger high-temperature area in the cylindrical swirl chamber are beneficial to the mixing of injected fuel and air.

Three-Dimensional Numerical Study of Fluid and Heat Transfer Characteristics of Dimpled Fin Surfaces

In the present study, a code based on the nonorthogonal curvilinear coordinates is developed with a collocated grid system generated by the two-boundary method. After validation of the code, it is used to compare simulated results for a fin-and-tube surface with coupled and decoupled solution methods. The results of the coupled method are more agreeable with the test data. Simulation for dimpled and reference plain plate fin-and-tube surfaces are then conducted by the coupled method within a range of inlet velocity from 1.0 m/s to 5 m/s. Results show that at identical pumping power the dimpled fin can enhance heat transfer by 13.8–30.3%. The results show that relative to the reference plain plate fin-and-tube surface, heat transfer rates and pressure drops of the dimpled fin increase by 13.8%–30.3% and 31.6%–56.5% for identical flow rate constraint. For identical pumping power constraint and identical pressure drop constraint, the heat transfer rates increase by 11.0%–25.3% and 9.2%–22.0%, respectively. By analyzing the predicted flow and temperature fields it is found that the dimples in the fin surface can improve the synergy between velocity and fluid temperature gradient.

Simulation Study on Horizontal Continuous Casting of Round Copper Billet with Electromagnetic Stirring

In this paper, a comprehensive three dimensional mathematical model is built to investigate the effect of electromagnetic stirring (EMS) on continuous casting process of copper round billet. The electromagnetic field is simulated by ANSYS software and the thermal-flow field is simulated by FLUENT software. The coupling between electromagnetic field and thermal-flow field is implemented by user-defined subroutines. The simulation results have good agreement with the experiment ones. The results show that electromagnetic frequency and current intensity have significant influence on the fluid velocity, temperature gradient and sump depth. The optimum current intensity and frequency are found to be 40A and 10Hz respectively.

Thermodynamic model for heterogeneous bubble nucleation in a temperature gradient

A thermodynamic model for heterogeneous bubble nucleation was developed to include the effect of the fluid temperature gradient next to a heated wall. An increase of the wall temperature gradient (heat flux) is shown to suppress bubble nucleation or increase the superheat needed for nucleation. Bubble nucleation is completely suppressed above a critical temperature gradient. The onset of nucleate boiling is shown to be related to the intersection of the qw″−Tw curve of the boiling system and the qw,c″−Tw curve of the fluid. The theoretical predictions agree with experimental results.

Silver–antimony deposits of Central Asia: physico-chemical model of formation and sources of mineralisation

Questions on the origin of Ag–Sb deposits are considered for several large economic ore districts in Central Asia: southeast Pamir (Tajikistan), Talas in northern Tien Shan (Kyrgyzstan), southeast Altai and northwest Mongolia, and Verkhoyansk province (Yakutia). The Ag–Sb mineralisation of Central Asia is spatially and temporally associated with alkaline mafic magmatism, and occasionally with coeval granitoid magmatism. Ag–Sb deposits have close spatial and temporal relationships with granitoid and alkaline mafic magmatism. The Ag–Sb deposits were formed in the temperature range 50–280°C, from Ag-specific reduced two-phase hydrothermal fluids containing a highly concentrated chloride solution (NaCl > FeCl2 > CaCl2 > KCl > MnCl2) and a high-density CO2+ CH4 ± N2 gaseous phase. Fluid salinities range widely from 40 to 0.1 wt% NaCl equiv. Geochemical specialisation of Ag–Sb ore is caused by chemical features of chloride brines enriched in Ag, Sb, Pb and Cu, with a relatively low Au content (≤10−6 mol/kg) within a pH of 3.5–4. The highest temperatures, concentrations of solutions and ore elements in the ore fluids were found in early quartz of siderite–tetrahedrite veins from below-ore level of the ore-forming system. Vertical zonation of the Ag–Sb mineralisation is caused by precipitation of different mineral associations from the same solution with temperatures decreasing towards the upper parts of the system. Sufficient vertical range of mineralisation (600–800 m) and relatively low Ag contents in the ore (300–600 ppm) result from the temperature decrease and absence of geochemical barriers. Higher temperature gradient and dilution of magmatogenic fluid with meteoric water was a geochemical barrier for the Ag-rich ore formation in a narrow (about 400 m) main ore deposition zone. Lateral zonation of Ag–Sb mineralisation is related to a staged development of the ore-forming process and deposition of different stage mineral associations at the beginning and end of the hydrothermal process. The Ag–Sb deposits are polygenic in terms of sources of ore elements and ore-forming fluids. Ore elements may have been inherited from magmatic and terrigenous host-rocks of different ages. Temporal correlation of the Ag–Sb mineralisation with alkaline mafic magmatism, He isotope composition and Hg contents in the ores confirm participation of a mantle source in the formation of the Ag–Sb deposits.

Fundamental Mechanism of Enhancing Single-phase Convective Heat Transfer

The recent developments in the study on heat transfer enhancement are reviewed and summarized. These include: The field synergy principle has been extended from parabolic flow to elliptic flow which is more often than not encountered in engineering. ② Validation of the field synergy principle has been performed via numerical and experimental methods for a number of cases, especially a specific test rig has been designed to demonstrate that when the flow direction is normal to the fluid temperature gradient, the flow and its velocity has nothing to do with the heat transfer in that direction. ③ It has been demonstrated via numerical examples that the field synergy principle can unify all the existing three explanations of convective heat transfer, hence it is the most fundamental mechanism for enhancing single-phase convective heat transfer. ④ Two examples of enhanced heat transfer configurations are presented which are deigned by applying the filed synergy principle and are characterized by the fact that the enhanced ratio of heat transfer is larger than the ratio of friction factor increase compared with the reference configuration. Finally some research needs in the filed synergy principle are proposed.

Numerical Simulation of Flow and Heat Transfer Characteristics Outside a Periodically Vibrating Tube

Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 140° in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.

Perturbation solution for a third-grade fluid flowing between parallel plates

The flow of non-Newtonian fluid in between two parallel plates at different temperatures is considered. A third-grade fluid with temperature-dependent viscosity is considered in the analysis and the Reynolds model used to account for it. Approximate analytical solutions for the velocity and temperature profiles are found using perturbation techniques. It is found that the influence of the non-Newtonian parameter and viscosity index is more pronounced in the region of the plate surfaces where the rate of fluid strain and temperature gradients are high.