Higher Nusselt Number Research Articles

Local heat transfer of vortex cooling with multiple tangential nozzles in a gas turbine blade leading edge cooling passage

Based on combined Infrared Thermography and numerical simulations, the heat transfer and flow performance in a vortex cooling configuration with 5 nozzles were investigated at various Reynolds numbers and temperature ratios. Validation was conducted between experiment and numerical simulation and results showed that the k-ω turbulence model has the best accuracy to predict the vortex cooling behavior. High Nusselt number regions appear under each nozzle in all cases, but air from downstream nozzles is easily impacted because of the “anti-cross flow” ability. Along the axial direction, targeting surface has a fluctuant heat transfer coefficient distribution, while along the circumferential direction, the heat transfer coefficient decreases linearly. Overall heat transfer coefficient increases with increasing Reynolds number and decreasing temperature ratio. Especially, the influence of temperature ratio under 3 setting methods is compared and discussed in detail, revealing the fundamental impact of temperature ratios. A globally-averaged Nusselt number correlation is presented over 22,000 < ReD < 39,000, 0.88 < TR < 0.94, and results in this paper are compared with other vortex cooling systems.

Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

The present direct numerical simulation (DNS) study, the first of its kind, explores the effect that the location of a cylinder, immersed in the turbulent wake of a grid-element, has on heat transfer. An insulated single square grid-element is used to generate the turbulent wake upstream of the heated circular cylinder. Due to fine-scale resolution requirements, the simulations are carried out for a low Reynolds number. Three locations downstream of the grid-element, inside the production, peak and decay regions, respectively, are considered. The turbulent flow in the production and peak regions is highly intermittent, non-Gaussian and inhomogeneous, while it is Gaussian, homogeneous and fully turbulent in the decay region. The turbulence intensities at the location of the cylinder in the production and decay regions are almost equal at 11 %, while the peak location has the highest turbulence intensity of 15 %. A baseline simulation of heat transfer from the cylinder without oncoming turbulence was also performed. Although the oncoming turbulent intensities are similar, the production region increases the stagnation point heat transfer by 63 %, while in the decay region it is enhanced by only 28 %. This difference cannot be explained only by the increased approaching velocity in the production region. The existing correlations for the stagnation point heat transfer coefficient are found invalid for the production and peak locations, while they are satisfied in the decay region. It is established that the flow in the production and peak regions is dominated by shedding events, in which the predominant vorticity component is in the azimuthal direction. This leads to increased heat transfer from the cylinder, even before vorticity is stretched by the accelerating boundary layer. The frequency of oncoming turbulence in production and peak cases also lies close to the range of frequencies that can penetrate the boundary layer developing on the cylinder, and therefore the latter is very responsive to the impinging disturbances. The highest Nusselt number along the circumference of the cylinder is shifted 45 degrees from the front stagnation point. This shift is due to the turbulence-generating grid-element bars that result in the prevalence of intense events at the point of maximum Nusselt number compared to the stagnation point.

Application of fractal theory in the arrangement of truncated ribs in a rectangular cooling channel (4:1) of a turbine blade

Ribs are widely employed in internal cooling passages of turbine blades and other heat transfer equipment to enhance heat transfer by making cooling air flow through internal ribbed passages. From mathematics, a fractal is an abstract object used to simulate naturally occurring objects, which exhibits similar patterns and properties also at smaller scales. The evolving fractal structures of the continuous ribbed channels contains two sides truncated ribs, three sides truncated ribs, five sides truncated ribs and nine sides truncated ribs. The thermal performance of the ribbed channels is compared by taking both heat transfer and pressure drop into consideration. Steady Liquid Crystal Thermography (LCT) is employed to measure surface temperature and derive heat transfer coefficients over the ribbed surfaces in the tested channels. Turbulent flow details are presented by numerical calculations with an established turbulence model, i.e. the k-ω SST model. From the experimental results, it is found that fractal truncated ribs with smaller length scales have larger heat transfer at the low Reynolds numbers. On the other hand, the difference of Nusselt numbers for different cases can be negligible at the high Reynolds numbers. Generally, fractal truncated ribs with smaller length scales have a more uniform heat transfer distribution. The correlation between Nusselt number and Reynolds number are respectively Re0.69, Re0.64 and Re0.61 for case 2 (Two Sides Truncated ribs), case 3 (Three Sides Truncated Ribs) and case 4 (Five Sides Truncated Ribs). The distribution of the high Nusselt number regions is kept identical during the fractal truncated ribs evolving process, which is also valid for the flow field.

Numerical Simulation of Flow and Heat Transfer in Structured Packed Beds with Smooth or Dimpled Spheres at Low Channel to Particle Diameter Ratio

Packed beds are widely used in catalytic reactors or nuclear reactors. Reducing the pressure drop and improving the heat transfer performance of a packed bed is a common research aim. The dimpled structure has a complex influence on the flow and heat transfer characteristics. In the present study, the flow and heat transfer characteristics in structured packed beds with smooth or dimpled spheres are numerically investigated, where two different low channel to particle diameter ratios (N = 1.00 and N = 1.15) are considered. The pressure drop and the Nusselt number are obtained. The results show that, for N = 1.00, compared with the structured packed bed with smooth spheres, the structured packed bed with dimpled spheres has a lower pressure drop and little higher Nusselt number at 1500 &lt; ReH &lt; 14,000, exhibiting an improved overall heat transfer performance. However, for N = 1.15, the structured packed bed with dimpled spheres shows a much higher pressure drop, which dominantly affects the overall heat transfer performance, causing it to be weaker. Comparing the different channel to particle diameter ratios, we find that different configurations can result in: (i) completely different drag reduction effect; and (ii) relatively less influence on heat transfer enhancement.

Flow structure and heat transfer of a sweeping jet impinging on a flat wall

While an impinging jet has been widely investigated because of its remarkable convective heat transfer performance, the impingement of a sweeping jet which undergoes periodic oscillation has drawn little interest. We experimentally examined the heat transfer of a sweeping jet impinging on a flat wall for several Reynolds number and nozzle-to-plate spacings and discovered unsteady flow structure to characterize heat transfer capability. Local Nusselt number on the wall was evaluated by measuring temperature with thermocouples, and a flow was visualized quantitatively using particle image velocimetry. The distribution of the Nusselt number is different from that of a round jet, exhibiting two distinct regions. Near the center of a sweeping jet, a high Nusselt number zone is formed without a noticeable peak commonly observed in a round jet. Away from the central region, the Nusselt number decreases monotonically. The trends of the Nusselt number at the two regions are correlated with the first mode of the flow structure obtained by proper orthogonal decomposition (POD). The boundary of the two regions is a local minimum of the first POD mode near the wall, and the magnitude of the first POD mode is large in the central region of high Nusselt number. It was also found that the distributions of mean lateral velocity and lateral velocity fluctuation were clearly different between the two regions, which implies that both quantities should be considered for the analysis of heat transfer performance.

Numerical study on thermal performance of an air-cooled heat exchanger: Effects of hybrid nanofluid, pipe arrangement and cross section

This study is a numerical investigation of a heat exchanger under external flow. Temperature variable properties are used to solve the governing equations. Air is selected as an external fluid and water and MgO-MWCNTs/EG hybrid nanofluid are selected as radiator fluid. In this paper, the efficiency of the horizontal and vertical radiator are studied using experimental data for thermo-physical properties of MgO-MWCNTs/EG Nanofluid. Then, tubes with circular and elliptic sections are investigated for different flows, inlet temperatures and concentrations. The pressure drop and Nusselt number for each simulation are analyzed. Results are presented in terms of temperature distribution, pressure drop, Nusselt number and their sensitivity analysis for different states. The results indicate that radiators with vertical tubes have better efficiency up to 10% than radiators with horizontal tubes. In addition, heat exchangers with circular pipes have 25% lower pressure drop and those with elliptical tubes had 10% higher Nusselt number. The finding shows that an increase in the Nano-fluid concentration improves the Nusselt number and increases the pressure drop.

Numerical Study of Heat Transfer Enhancement of Roll-to-Roll Microchannel Heat Exchangers

The heat transfer performance of two roll-to-roll microchannel heat exchangers with square cross section and side length ranging from 0.2 mm to 0.5 mm were investigated via numerical studies. In order to assess the heat transfer enhancement, equivalent straight channel heat exchangers were also researched numerically as comparisons. For the roll-to-roll devices, numerical studies demonstrated that there were two reasons for heat transfer enhancement. First, when the average Dean number of the fluid was greater than approximately 10, Dean vortices started to form within the roll-to-roll microchannels, enhancing the convective heat transfer between channels. Second, the compact roll-to-roll structure of the heat exchangers increased the area of heat transfer compared with straight microchannel equivalents, and thus promoted the conductive heat transfer. Numerical simulations noted both higher Nusselt numbers and higher thermal performance factors (TPF) for roll-to-roll microchannel heat exchangers compared with equivalent straight channels and were employed to optimize both the microchannel cross section dimensions and the wall thickness between channels. In addition, the swirling strength and the heat transfer area were also calculated to characterize the convective and conductive heat transfer, respectively, allowing for a comparison between two roll-to-roll microchannel heat exchanger designs.

The Thermal Hydraulic Performance of Wavy PCHE with Different Materials and Geometric Parameters

The printed circuited heat exchanger (PCHE) contain several different channel configurations, such as straight channel, zigzag channel and wavy channel. The wavy channel has better thermal performance than the straight channel and better hydraulic performance than the zigzag channel. This paper explores the thermal hydraulic performance of wavy channel PCHE. The numerical analysis of the PCHE in different materials and geometric parameters are conducted by computational fluid dynamics (CFD) tool. The materials applied in simulations involve Alloy617, Titanium Grade 3, Carlson 2205, UNS S30400 and Sandvik 253A. The results show that the materials have little effect on the thermal-hydraulic performance. The geometric parameters include channel degree varying from 10°to 50°, channel amplitude varying from 1mm to 5mm and the radium of hot/cold channel varying from 0.4mm to 2.0mm. It is found that the larger radium of hot channel results out the lower Nusselt number and lower fanning-friction factor while the higher radium of cold channel produces the higher Nusselt number and lower fanning-friction factor. The larger channel amplitude indicates the higher fanning-friction factor and lower Nusselt number. The larger channel degree indicates the higher fanning-friction factor, and higher Nusselt number.

Turbulence Combined Convective Heat Transfer and Nanofluids Flows over Double Forward Facing Steps

Predictions are reported for mixed convection using various types of nanofluids over forward-facing double steps in a duct. The continuity, momentum and energy equations are discretized and the simple algorithm is applied to link the pressure and flow fields inside the domain. Different types of nanoparticles Al2O3, CuO, SiO2 and ZnO, with different volume fractions in range of 1-4% are investigated to identify their effects on the heat transfer and fluid characteristics. Numerical investigations are conducted using finite volume method. The results indicate that SiO2 -water has the highest Nusselt number followed by Al2O3-water, CuO -water and ZnO-water. The Nusselt number increases as the volume fraction increases but decreases as the nanoparticles diameter increases.

Hydrothermal optimization of SiO2/water nanofluids based on attitudes in decision making

In order to avoid the costs of experimental evaluations, soft computing methods like artificial neural network (ANN) and genetic algorithm have remarkably grabbed the attentions of investigators for predicting the hydrothermal characteristics of different types of nanofluids. In this paper, the implementation of ANN and genetic algorithm for modeling and multi-criteria optimizing the hydrothermal behavior of SiO2/water nanofluid has been investigated. Using the data obtained from the experimental analysis, an ANN model is developed to estimate the pressure drop and Nusselt number as a function of volume concentration, Reynolds number, and inlet temperature. Different network structures were assessed and it has been achieved that a network with 2 hidden layers and 6 neurons in every layer provides the most accurate prediction. It is revealed that the developed network is satisfactorily accurate to determine the Nusselt number and pressure drop of SiO2/water nanofluid compared to the empirical correlations. To optimize the hydrothermal behavior of the nanofluid (i.e. to find the optimal cases with highest Nusselt number and the relatively least pressure drop), the genetic algorithm coupled with compromise programming approach has been implemented considering decision maker's attitude.

Investigations of flow structures and heat transfer in a swirl chamber with different inlet chambers and various aerodynamic parameters

This paper numerically compares the flow structures and thermal performance in a swirl chamber with three different inlet chambers. The cross section of inlet chambers are constant, convergent, and divergent along the streamwise direction, respectively. Parametric studies involve Reynolds number and the temperature ratio between cooling air and the wall of swirl chamber. Results indicate that shear layer vortices, corner vortices, and large-scale vortices appear in different sections of swirl chamber. Besides, Görtler vortices appear in convergent inlet chamber. The alternation of the inlet chamber changes more than the value of the Nusselt number. It also changes the flow structures and the distribution of Nusselt number. In comparisons, the swirl chamber with convergent inlet chamber possesses the highest thermal performance coefficient. With the increase of Reynolds number, the intensity of heat transfer is significantly enhanced. Different from former research, the rise of temperature ratio leads to relatively high Nusselt number due to the addition of multiple jet nozzles and the inlet chamber. In addition, empirical correlation among global average Nusselt number, Reynolds number, and temperature ratio is obtained to assist relevant research and design.

Flow structure and heat transfer characteristics of a dimpled wedge channel with a bleed hole in dimple at different orientations and locations

In this study, the heat transfer and friction factor characteristics of a dimpled surface with a bleed hole in the converge channel is studied using a numerical method. The converge angle of the channel is approximately 12.7°. The installation angle of the bleed hole ranges from −30° to 30°. The diameter of the bleed hole is 0.5 mm. The Reynolds number of the main flow is between 8500 and 60,000. The mass flow rate of each bleed hole is 0.5% of the total mass flow rate at the inlet. A dimple without a bleed hole is used as the Baseline. It is found that the bleed hole significantly destroys the low-speed-recirculation. The area of reverse region decreases and the strength of secondary vortex increases as the installation angle increases. The spiral point and reattachment line are also changed as the installation angle changes. This is more distinct for a bleed hole at α = 30°. The primary vortex and spiral point are almost invisible. Two regions with a high turbulent kinetic energy and a high Nusselt number are found. The area-averaged Nu number ratios increase as the installation angle increases. The thermal performance is also improved by the bleed hole.

Laminar Mixed Convective Nanofluid Flow in a Channel with Double Forward Facing Steps: A Numerical Simulation Study

Predictions are reported for mixed convection using various types of nanofluids overforward-facing double steps in a duct. The continuity, momentum and energy equations arediscretized and the simple algorithm is applied to link the pressure and flow fields inside the domain. Different types of nanoparticles Al2O3, CuO, SiO2 and ZnO, with different volume fractions in range of 1-4% and different nanoparticles diameter in the range of 20 – 80nm in base fluid (water) were used. Numerical investigations are conducted using finite volume method. In this study, different parameters such as the geometrical specifications (different steps heights in the range of h1= 0.01m-0.04m and h2 = 0.03m-0.06m for FFS) are used. Different Reynolds numbers in the range of 50-2000 (laminar flow) are investigated to identify their effects on the heat transfer and fluid characteristics. The results indicate that SiO2-water has the highest Nusselt number followed by -water, -water and ZnO-water. The Nusselt number increases as the volume fraction increases but it decreases as the nanoparticles diameter increases. The velocity magnitude increases as the density of nanofluids decreases. The recirculation region and the Nusselt number increase as the step height, Reynolds number, and the volume fraction increase.

Numerical investigation of local heat transfer distribution on surfaces with a non-uniform temperature under an array of impinging jets with various nozzle shapes

Numerical calculations of heat transfer characteristics of an impingement cooling system with a non-uniform temperature on a cooled surface using ANSYS CFX have been performed. The influence of a surface heat flux qw(x) and a nozzle shape on the Nusselt number distribution on the cooled surface has been studied. The setup consisted of a cylindrical plenum with an inline array of ten impingement jets. Cylindrical, convergent divergent shapes of nozzles and linear temperature distribution on the cooled surface have been considered for various heat fluxes qw(x). Results indicate that geometry of the cylindrical nozzles resulted in the highest Nusselt numbers along the cooled surface. The line of the averaged Nusselt number has a trend to increase in the direction of the flow for the cooling system with increasing values of the surface heat flux q(x). This tendency can be observed for all presented shapes of jets. On the other hand, for decreasing functions of the heat flux qw(x), the Nusselt number distribution is more uniform. It can be observed for all types of nozzles. Very similar values of the Nusselt number occur especially for the non-uniform heat flux 5000-2500W/m2. For constant values of the heat flux q(x) = 5000W/m2, the line of the average Nusselt number has a trend to increase slightly in the direction of the flow. Numerical analysis of different mesh density results in good convergence of the GCI index, what excludes mesh size dependency. The presented study is an extension of the paper (Marzec and Kucaba-Pietal, 2016) and aims at answering the question how the Nusselt number distribution on the cooled surface is affected by various geometries of nozzles for a non-uniform surface heat flux qw(x).

Natural convection of melted core at the bottom of nuclear reactor vessel in a severe accident

A hypothetical severe accident was investigated when nuclear fuels are melted. Experiments were performed for natural convection heat transfer of the melted core at the bottom of the reactor vessel. To achieve highly buoyant force, mass transfer experiment was adopted by using copper electroplating system based on the analogy concept. Mean and local Nusselt numbers were measured and compared for 2 different configurations of the melted core: 2-layer and 3-layer oxide pools. The phenomenological analysis was carried out comparing with 2 different oxide pool configurations of the present works and the existing studies. Heat ratio of the lower head to the top plate was affected by Prandtl number and geometrical difference. Compared with the existing studies, the Nu of lower head were about 40% lower, and those of the top plate were about 60% higher. The local Nusselt number ratio of the lower head gradually increased along the upper region and did not become affected by thermal conditions of the top plate. The local top plate Nusselt numbers for the 2D test rig were uniform, while those for the 3D test rig had steep slope with the highest Nusselt number at the center.

UNSTEADY NUMERICAL SIMULATION OF TURBULENT FORCED CONVECTION IN A RECTANGULAR PIPE PROVIDED WITH WAVED POROUS BAFFLES

Numerical simulations of flow and heat transfer in a serpentine heat exchanger configuration are presented to demonstrate application of porous media techniques in heat exchanger analyses. In this study, steady-state 3D turbulent forced convection flow and heat transfer characteristics in a rectangular pipe with baffles attached inside pipe have been numerically investigated under constant wall heat flux boundary condition. Numerical study has been carried out for Reynolds number of 20000-50,000, Prandtl number of 0.71, baffle distances h/D h of 1/3 and 2/3, and baffle thickness e of 1/12, 2/3. It is observed that rectangular pipe having waved baffles has a higher Nusselt number and friction factor compared to the smooth rectangular pipe without baffles. Periodically fully developed conditions are obtained after a certain module. Maximum thermal performance factor is obtained for the baffle waved. Results show that baffle distance, waved porous baffle, and Reynolds number play important role on both flow and heat transfer characteristics. All the numerical results are correlated within accuracy of ±2% and ±2.5% for average Nusselt number and friction factor, respectively.

Boundary stratum exploration of unsteady 3D MHD stagnation point flow of Al–Cu water nanofluid

A theoretical analysis of unsteady, three-dimensional stagnation-point flow of Al–Cu nanoparticle suspended water based nanofluid is performed. The thermophysical properties of alloy nanoparticles like density, specific heat capacity and thermal conductivity are computed using appropriate formula. A micro-convection based model framed by Patel et al. [1] is used for predicting the thermal conductivity of nanofluid. The governing physical model is mathematically modeled into a set of non-linear parabolic partial differential equations and solved by a shooting technique. A parametric study is performed for a varied range of nanoparticle volume fraction, ratio of the velocity gradient, unsteadiness parameter and the magnetic parameter. Additionally, the importance of the different compositional characteristics of the alloy nanoparticles on the flow and the thermal field is also examined. It is observed that the Nusselt number increases as the unsteadiness parameter λ decreases and the ratio of the velocity gradient c increases in the nodal point region. A novel result of the analysis reveals that the highest skin friction is obtained for Al50Cu50 - water and the highest Nusselt number is obtained for Al90Cu10-water.

Thermo-Fluidic Comparison between Sandwich Panels with Tetrahedral Lattice Cores Fabricated by Casting and Metal Sheet Folding

This numerical study compares single-phase forced convective heat transfer between two sandwich panels with tetrahedral metallic lattice cores separately fabricated by investment casting and the more cost-effective metal sheet folding method. The numerical model is validated against available experimental data. For a given Reynolds number and core porosity, the results reveal that the brazed sandwich outperforms the casted sandwich, exhibiting a 13% to 16% higher Nusselt number. Bigger vertexes and more evident blockage of mainstream by the ligaments are found to intensify the horseshoe vortex and the counter-rotating vortex pair upstream and downstream of each vertex. Relative to the casted sandwich panel, therefore, endwall heat transfer is enhanced by 22% to 27%, while similar heat transfer is achieved on the ligaments. It is also found that, for a given Reynolds number, the brazed sandwich induces a 1.6 to 1.7 times higher pressure drop relative to the casted sandwich due to more severe flow separation caused by the sharp edges of the rectangular ligaments. Finally, for a given pumping power, both sandwiches provide a similar heat transfer performance. Given that the brazed sandwich is more cost-effective and easier to fabricate than the casted one, the former may be superior from an engineering application point of view.

Numerical Investigation of Single-Phase Heat Transfer in a 2 × 2 Rod Bundle with Spacer Grids for a High Pressure Heat Transfer Facility

Numerical simulation has been widely used in nuclear reactor safety analyses to gain insight into key phenomena. This paper compares simulations of a single-phase steady flow in a 2 × 2 rod bundle with spacer grids among different codes based on the high pressure heat transfer facility at University of Wisconsin. The detailed computational fluid dynamics modeling methodology was developed using FLUENT to help in the facility design and pretest analyses. After comparison between different turbulence models, the Standard k-ω was chosen to simulate the effect of unheated solid walls and grid spacers. It was found that solid walls had a small influence on the flow and heat transfer behavior. We note the effect of rod-to-wall gap needs be taken into account if it is larger than half of the gap between the rods. We compared the simulations of FLUENT, COBRA-TF, and TRACE to determine the position of thermocouples to be used in the planned experiments. An investigation was performed on the effect of bending angles of the grid spacer mixing vanes. Results showed that a larger bending angle results in higher turbulence mixing and locally higher Nusselt numbers downstream of the mixing vanes. Also, a small change of the bending angles results in a notable difference in the temperature distributions of the main flow.

Numerical investigation and optimization of heat transfer and exergy loss in shell and helical tube heat exchanger

In this research paper, numerical model of shell and helical tube heat exchanger is investigated to assess heat transfer coefficient and exergy loss. Four design parameters including pitch coil, tube diameter, hot and cold flow rate, which are more significant for the heat exchanger performance, were taken to consideration. Then, Taguchi approach is applied to figure out the optimum levels of the design factors. Sixteen cases with diverse design parameters are modeled and analyzed numerically. The results are indicated that tube diameter and cold flow rate are the most significant design parameters of heat transfer and exergy loss, respectively. Furthermore, the highest Nusselt number are achieved by more both cold and hot flow rates and also, heat transfer coefficient are reduced by increasing of pitch coil as well as by increasing of hot flow rate, the exergy loss increases. The optimum levels for heat transfer coefficient are: pitch 13mm, tube diameter 12mm, cold and hot flow rate 4LPM. Furthermore, the optimum level for exergy loss are: pitch 13mm, tube diameter 12mm, cold and hot flow rate 1LPM.