Fluid Flow and Heat Transfer over Staggered ꞌ+ꞌ Shaped Obstacles

Heat and fluid flow in a rectangular channel heat sink equipped with longitudinal vortex generators have been numerically investigated in the range of Reynolds numbers between 25 and 200. Aqueous solutions of carboxymethyl cellulose (CMC) with different concentrations (200–2000ppm), which are shear-thinning non-Newtonian liquids, have been utilised as working fluid. Three-dimensional simulations have been performed on a plain channel and a channel with five pairs of vortex generators. The channels have a hydraulic diameter of 8mm and are heated by constant wall temperature. The vortex generators have been mounted at different angles of attack and locations inside the channel. The shear-thinning liquid flow in rectangular channels with longitudinal vortex generators are described and the mechanisms of heat transfer enhancement are discussed. The results demonstrate a heat transfer enhancement of 39–188% using CMC aqueous solutions in rectangular channels with LVGs with respect to a Newtonian liquid flow (i.e. water). Additionally, it is shown that equipping rectangular channels with LVGs results in an enhancement of 24–135% in heat transfer performance vis-à-vis plain channel. However, this heat transfer enhancement is associated with larger pressure losses. For the range of parameters studied in this paper, increasing the CMC concentration, the angle of attack of vortex generators and their lateral distances leads to an increase in heat transfer performance. Additionally, heat transfer performance of rectangular channels with longitudinal vortex generators enhances with increasing the Reynolds number in the laminar flow regime.

Microchannels are of current interest for use in heat exchangers where very high heat transfer performance is desired. Microchannels provide very high heat transfer coefficients because of their small hydraulic diameters. In this study, an investigation of fluid flow and heat transfer in microchannels is conducted. A review of the literature published regarding fluid flow and heat transfer in microchannels is provided in this study. A thorough background on the theory of internal convective heat transfer is provided as well. A critical analysis of some of the published heat transfer experiments on microchannels is also given. It was found that some of the experimental methods published recently employ experimental and data reduction techniques which may result in errors for reported Nusselt numbers. A brief computational fluid dynamics (CFD) investigation into the heat transfer and fluid flow performance of some channels with designed bumps is also presented. It was found that designed channels offer enhanced Nusselt numbers in the turbulent regime at the cost of a higher friction factor when compared to a plain channel. Fluid flow and heat transfer experiments were conducted on a copper microchannel heat exchanger (MHE). An experimental method of imposing a constant surface temperature to the MHE was used. A description of the experimental apparatus and procedure is provided. The friction factor results from the experiments agree fairly well with theoretical correlations. The experimental Nusselt number results agree with theory very well in the transition/turbulent regime, but the results show a higher Nusselt number in the laminar regime than predicted by theoretical correlations. A full description of the data reduction and analysis of the experimental data is given. A CFD model was created to simulate the fluid in the inlet plenum and the microchannels. The results from these simulations are presented, and they show good agreement with the experimental data in the transition/turbulent regime as well as with theoretical correlations for laminar and turbulent flow.

The high rates of heat transfer obtained by using micro or mini channels makes them an attractive alternative to conventional methods of heat dissipation, especially in microelectronics cooling system. In this paper, three dimensional fluid flow and heat transfer phenomena for different coolant in rectangular, elliptic and triangular miniscale channels is investigated. In all computational regions this paper improved the Boussinesq and the thermal physical properties of the coolant are temperature dependent. The heat transfer Nusselt numbers and flow resistance coefficients of different channels are obtained, and the pressure drop and the temperature increasing of the coolant in three mini channels are studied. The numerical simulation shows that the mini rectangular channels’ heat transfer and flow resistance characteristics are optimal.

Thermal and hydraulic performance of natural convection in a rectangular storage cavity

Purpose – The employment of continuous ribs in a passage involves a noticeable pressure drop penalty, while other studies have shown that truncated ribs may provide a potential to reduce the pressure drop while keeping a significant heat transfer enhancement. The purpose of this paper is to perform computer-aided simulations of turbulent flow and heat transfer of a rectangular cooling passage with continuous or truncated 45-deg V-shaped ribs on opposite walls. Design/methodology/approach – Computational fluid dynamics technique is used to study the fluid flow and heat transfer characteristics in a three-dimensional rectangular passage with continuous and truncated V-shaped ribs. Findings – The inlet Reynolds number, based on the hydraulic diameter, is ranged from 12,000 to 60,000 and a low-Re k-e model is selected for the turbulent computations. The local flow structure and heat transfer in the internal cooling passages are presented and the thermal performances of the ribbed passages are compared. It is found that the passage with truncated V-shaped ribs on opposite walls provides nearly equivalent heat transfer enhancement with a lower (about 17 percent at high Reynolds number of 60,000) pressure loss compared to a passage with continuous V-shaped ribs or continuous transversal ribs. Research limitations/implications – The fluid is incompressible with constant thermophysical properties and the flow is steady. The passage is stationary. Practical implications – New and additional data will be helpful in the design of ribbed passages to achieve a good thermal performance. Originality/value – The results imply that truncated V-shaped ribs are very effective in improving the thermal performance and thus are suggested to be applied in gas turbine blade internal cooling, especially at high velocity or Reynolds number.

The Pebble Bed Modular Reactor (PBMR), a promising Generation IV nuclear reactor design, raises many novel technological issues for which new experience and techniques must be developed. This brief study explores a few of these issues, utilizes a computational fluid dynamics code to model some simple phenomena, and points out deficiencies in current knowledge that should be addressed by future research and experimentation. A highly simplified representation of the PBMR core is analyzed with FLUENT, a commercial computational fluid dynamics code. The applied models examine laminar and turbulent flow in the vicinity of a single spherical fuel pebble near the center of the core, accounting for the effects of the immediately adjacent fuel pebbles. Several important fluid flow and heat transfer parameters are examined, including heat transfer coefficient, Nusselt number, and pressure drop, as well as the temperature, pressure, and velocity profiles near the fuel pebble. The results of these “unit cell” calculations are also compared to empirical correlations available in the literature. As FLUENT is especially sensitive to geometry during the generation of a computational mesh, the sensitivity of code results to pebble spacing is also examined. The results of this study show that while a PBMR presents a novel and complex geometry, a code such as FLUENT is suitable for calculation of both local and global flow characteristics, and can be a valuable tool for the thermal-hydraulic study of this new reactor design. FLUENT results for pressure drop deviate from the Darcy correlation by several orders of magnitude in all cases. When determining the heat transfer coefficient, FLUENT is again much lower than Robinson’s correlation. Results for Nusselt number show better agreement, with FLUENT predicting results that are 10 or 20 times as large as those from the Robinson and Lancashire correlations. These differences may arise because the empirical correlations concern mainly integral parameters, while the FLUENT model focuses on local flow behaviors. Local phenomena are significant in the case of local heat transfer characteristics, fine temperature distribution calculations to identify hot spots, and fission product transport phenomena. All of these are important to a safety analysis of the PBMR reactor during normal operation, as well as during transient circumstances, and should be the focus of future research efforts.

Heat transfer in BCC lattice materials: Conduction, convection, and radiation

Fluid flow and heat transfer in a nanochannel may depart from the traditional behavior due to the scale effect, and the velocity slip and temperature jump at the fluid-solid interface must be taken into account. A lot of papers about fluid flows in nanochannels with the same wettability at two surfaces have been published. It is necessary to investigate fluid flow and heat transfer in nanochannels with the asymmetric wettability by the molecular dynamics method. The fluid velocity and temperature distributions, interfacial velocity slip and temperature jump in a rough nanochannel are evaluated. The effects of asymmetric wettability on the velocity slip, temperature jump and internal fluid heat transfer are analyzed. The results indicate that the velocity of the fluid flow under an external force in a nanochannel in a bulk region is of a parabolic distribution, but the parabolic distribution is not centrosymmetric because of the centrosymmetric density profile. The difference in density distribution can affect the fluid flow. Viscous dissipation due to shear flow will increase the fluid temperature. The range that is affected by the interaction between solid and liquid is small. So the wettability of the cold wall hardly affects the velocity of the fluid near the hot wall, and the slip velocity is almost constant. At this time, the negative slip will take place at the fluid-solid interface near the hot wall. But the velocity of the fluid near the cold wall comes up with the increasing hydrophobicity of the cold wall, and the slip velocity increases. The temperature jump on both sides of interface increases with the increasing hydrophobicity of the cold wall, but the degree of temperature jump at a liquid-cold solid interface is higher than that at a liquid-hot solid interface. Then the fluid temperature near the cold wall gradually exceeds the fluid temperature near the hot wall. The internal heat flow of the fluid will be reversed. The inverted temperature profile of the fluid will appear. The inverted temperature profile becomes more obvious when the degree of asymmetric wettability increases.

Numerical simulation of free convection of nanofluid in a square cavity with an inside heater

Purpose The purpose of this paper is to develop a combined method for three-dimensional incompressible flow and heat transfer by the spectral collocation method (SCM) and the artificial compressibility method (ACM), and further to study the performance of the combined method SCM-ACM for three-dimensional incompressible flow and heat transfer. Design/methodology/approach The partial differentials in space are discretized by the SCM with Chebyshev polynomial and Chebyshev–Gauss–Lobbatto collocation points. The unsteady artificial compressibility equations are solved to obtain the steady results by the ACM. Three-dimensional exact solutions with trigonometric function form and exponential function form are constructed to test the accuracy of the combined method. Findings The SCM-ACM is developed successfully for three-dimensional incompressible flow and heat transfer with high accuracy that the minimum value of variance can reach. The accuracy increases exponentially along with time marching steps. The accuracy is also improved exponentially with the increasing of nodes before stable accuracy is achieved, while it keeps stably with the increasing of the time step. The central processing unit time increases exponentially with the increasing of nodes and decreasing of the time step. Research limitations/implications It is difficult for the implementation of the implicit scheme by the developed SCM-ACM. The SCM-ACM can be used for solving unsteady impressible fluid flow and heat transfer. Practical implications The SCM-ACM is applied for two classic cases of lid-driven cavity flow and natural convection in cubic cavities. The present results show good agreement with the published results with much fewer nodes. Originality/value The combined method SCM-ACM is developed, firstly, for solving three-dimensional incompressible fluid flow and heat transfer by the SCM and ACM. The performance of SCM-ACM is investigated. This combined method provides a new choice for solving three-dimensional fluid flow and heat transfer with high accuracy.

A porous model for the square pin-fin heat sink situated in a rectangular channel with laminar side-bypass flow

Perioperative hypothermia is a common problem in anaesthesia. To maintain a constant body heat content it is necessary to achieve a steady state of metabolic heat production and external heat exchange. During anaesthesia and surgery this balance is disturbed. The heat production is decreased to a level below resting metabolic heat production while heat losses increase due to the surgical procedure. As a consequence, heat losses exceed metabolic heat production and patients get hypothermic. Even during large abdominal operations the major source of heat loss (about 85%) is the radiative and convective heat loss from the skin. Because during anaesthesia heat production cannot participate in regulation, heat loss is of major importance. The four mechanisms of heat exchange are convection, conduction, radiation and evaporation. While evaporation is proportional to the difference of partial pressure of water vapour, the heat exchange by convection, conduction and radiation can be described as follows: Q ˙ A −1 = h( T Skin − T Env); Q ˙ : heat flux (W), A: area (m 2); h: heat exchange coefficient (W m −2 °C −1); T Skin: skin temperature (°C); T Env: environmental temperature (°C). The driving force of heat exchange is the temperature difference. The heat exchange coefficient describes the efficacy of heat exchange for a given temperature difference. For operating room conditions the heat exchange coefficient for radiation and convection can be combined ( h RC). To determine the heat exchange coefficient for a given heat exchange mechanism the temperature gradient (Δ T) between environment and skin has to be varied and the resulting heat flux per unit area (W m −2) measured by heat flux transducers. The heat exchange coefficient can be determined by linear regression analysis as the slope of the heat flux per unit area versus the temperature gradient. Several measures have been established to prevent perioperative hypothermia. Insulation reduces heat loss from skin about 30–80% depending on the material. Insulation reduces the h RC and is described as 1/ h RC. Heat transfer from the environment to the body can be achieved by active warming as forced air warming, conductive warming or radiative warming devices. The related heat flux is influenced by the h of each system, the temperature gradient between the skin and the blanket and the treated area. If the heat exchange coefficient is known, the heat exchange can be predicted by the measurement of the temperature gradient. This provides an estimation of intraoperative heat exchange, where heat flux measurement is difficult or impossible.

The study of stagnation point flow and heat transfer over a stretching/shrinking sheet in a hybrid nanofluid has numerous possible applications such as microelectronics cooling design and, heat transfer in atmospheric re-entry. This study needs to solve the governing mathematical model (partial differential equations) to explore the properties of fluid flow and heat transfer. The model is converted into a system of ordinary differential equations by using similarity variables. In order to solve these nonlinear ordinary differential equations, the study uses numerical collocation method of Haar wavelets. By employing this method, the study can forecast several significant fluid flow and heat transfer characteristics, such as the local Nusselt number, skin friction coefficient, and temperature and velocity profiles. The numerical results show that dual solutions can appear for certain values of the shrinking parameter. It is also noted that the slip parameter, suction/injection parameter, and nonlinear parameter contribute to widening the range of possible solutions to allow for more variations in the fluid behaviour under different conditions.

Nanofluid, the fluid suspensions of a metallic nanoparticle, became a coolant fluid that is used when a promising enhancement in heat transfer is required. In the current study, the characteristics of fluid flow and heat transfer are numerically investigated using different nanofluids (Al2O3–H2O, TiO2–H2O, and SiO2–H2O) and different micro-channel heat sink (MCHS) configurations (rectangular, triangular, trapezoidal, and circular). In this numerical investigation, the effect of Re number ranged from 890 to 1500, and the effect of nanoparticle concentration ranged from 1% to 7% at constant heat flux q = 106 W/m2, and constant fluid inlet temperature of 288 K, were studied. The average heat transfer coefficient, h, and pressure drop, Δp, are used to quantify the fluid flow and heat transfer characteristics in each MCHS configuration and for each nanoparticle concentration. It is revealed that a better heat transfer coefficient is obtained for Al2O–H2O compared with other types of nanoparticles and pure water, such as 8.58% heat transfer coefficient improvement obtained at Re = 1500 and φ=7% more than that of pure water. It is also inferred that the maximum heat transfer coefficient is obtained by the triangular MCHS; however, it has the highest pressure drop because of the lowest hydraulic diameter.

Heat transfer in laminar plane channel flow with uniform suction or injection