Near-Wall Focusing of Phase Change Micro-Particles for Local Heat Transfer Enhancement

2011 ◽  
Author(s):  
Andrej Lenert ◽  
Youngsuk Nam ◽  
Matthew W. Thoms ◽  
Bekir S. Yilbas ◽  
Evelyn N. Wang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nusselt Number ◽  
Phase Change ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heated Wall ◽  
Channel Height ◽  
Micro Particles ◽  
Particle Layer

We report the effect of confining micron-sized phase-change particles to a layer near the heated wall of a parallel plate channel. We developed a numerical model which assumes fully-developed laminar flow and a constant heat flux applied to one wall. Melting of the confined phase-change particles is incorporated in the model using a spatially-dependent and temperature-dependent effective heat capacity. We investigated the effect of channel height, height of the phase-change particle layer, heat flux, and fluid properties on the peak local Nusselt number (Nu*) and the averaged Nusselt number over the melting length (Numelt). Compared to the base Nusselt number for this geometry (Nuo = 5.385), Numelt and Nu* enhancements were determined to be as high as 15% and 45%, respectively. For a constant mass fraction of particles in the phase-change layer, Numelt is optimized when the phase-change particles are confined to within 35% of the channel width. These studies suggest a strategy to enhance heat transfer with phase change particles for various thermal-fluidic systems.

scholarly journals Steady interaction of a turbulent plane jet with a rectangular heated cavity

2016 ◽  
Vol 20 (5) ◽  
pp. 1485-1498
Author(s):  
Farida Iachachene ◽  
Amina Mataoui ◽  
Yacine Halouane
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Flow Structure ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heated Wall ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Potential Core

Turbulent heat transfer between a confined jet flowing in a hot rectangular cavity is studied numerically by finite volume method using the k-w SST one point closure turbulence model. The location of the jet inside the cavity is chosen so that the flow is in the non-oscillation regime. The flow structure is described for different jet-to-bottom-wall distances. A parametrical study was conducted to identify the influence of the jet exit location and the Reynolds number on the heat transfer coefficient. The parameters of this study are: the jet exit Reynolds number (Re, 1560< Re <33333), the temperature difference between the cavity heated wall and the jet exit (DT=60?C) and the jet location inside the cavity (Lf, 2? Lf? 10 and Lh 2.5<Lh?10). The Nusselt number increased and attained its maximum value at the stagnation points and then decreased. The flow structure is found in good agreement with the available experimental data. The maximum local heat transfer between the cavity walls and the flow occurs at the potential core end. The ratio between the stagnation point Nusselt numbers of the cavity bottom (NuB0) to the maximum Nusselt number on the lateral cavity wall (NuLmax) decreased with the Reynolds number for all considered impinging distances. For a given lateral confinement, the stagnation Nusselt number of the asymmetrical interaction Lh?10 is almost equal to that of the symmetrical interaction Lh=10.

scholarly journals Experimental investigation of flow and heat transfer characteristics on matrix ribbed channel

2020 ◽  
Vol 24 (3 Part A) ◽  
pp. 1593-1600
Author(s):  
Bo Zhang ◽  
Quan Hong ◽  
Yuanyuan Dou ◽  
Honghu Ji ◽  
Rui Chen
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Rectangular Channel ◽  
Turbine Blades ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Channel Height ◽  
Height Ratio ◽  
Ribbed Channel ◽  
The Matrix

The effect of the rib width to height ratio t/e and width to pitch ratio t/p on the local heat transfer distribution in a rectangular matrix ribbed channel with two opposite in line 45? ribs are experimentally investigated for Reynolds numbers from 54000 to 150000. The rib height to channel height ratio e/H is 0.5, t/p and t/e both varies in range of 0.3-0.5. To simulate the actually situation in turbine blades, and provide useful direct results for turbine blade designers, the parameters are same with the blade. The experiments results show that, in comparison to fully developed flow in a smooth pipe of equivalent hydraulic diameter, the Nusselt number inside the matrix-ribbed rectangular channel is increased up to 5 to 9 times higher, while total pressure drop is enlarged by up to significant magnitude. The Nusselt number ratio increases with t/p and t/e increased. Semi-empirical heat transfer is developed for designing of cooling channel.

A New Nusselt Number for Complicated Configurations

2013 ◽  
Author(s):  
Tom I-Ping Shih ◽  
Srisudarshan Krishna Sathyanarayanan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nusselt Number ◽  
Local Heat ◽  
Negative Slope ◽  
Test Problems ◽  
Rectangular Duct ◽  
Circular Duct ◽  
Flow And Heat Transfer ◽  
Local Heat Flux

Convective heat transfer over surfaces is generally presented in the form of the heat-transfer coefficient (h) or its nondimensional form, the Nusselt number (Nu). Both require the specification of the free-stream temperature (Too) or the bulk (Tb) temperature, which are clearly defined only for simple configurations. For complicated configurations with flow separation and multiple temperature streams, the physical significance of Too and Tb becomes unclear. In addition, their use could cause the local h to approach positive or negative infinity if Too or Tb is nearly the same as the local wall temperature (Twall). In this paper, a new Nusselt number, referred to as the SCS number, is proposed, that provides information on the local heat flux but does not use h and hence by-passes the need to define Too or Tb. CFD analysis based on steady RANS with the shear-stress transport model is used to compare and contrast the SCS number with Nu for two test problems: (1) compressible flow and heat transfer in a straight duct with a circular cross section and (2) compressible flow and heat transfer in a high-aspect ratio rectangular duct with a staggered array of pin fins. Parameters examined include: Reynolds number at the duct inlet (3,000 to 15,000 for the circular duct and 15,000 and 150,000 for the rectangular duct), wall temperature (Twall = 373 K to 1473 K for the circular duct and 313 K and 1,173 K for the rectangular duct), and distance from of the inlet of the duct (up to 100D for the circular duct and up to 156D for the rectangular duct). For the circular duct, Nu was found to decrease rapidly from the duct inlet until reaching a minimum and then to rise until reaching a nearly constant value in the “fully” developed region if the wall is heating the gas. If the wall is cooling the gas, then Nu has a constant positive slope in the “fully” developed region. The location of the minimum in Nu and where Nu becomes nearly constant in value or in slope are strong functions of Twall. For the SCS number, the decrease from the duct inlet is monotonic with a negative slope, whether the wall is heating or cooling the gas. Also, different SCS curves for different Twall approach each other as the distance from the inlet increases. For the rectangular duct, Nu tends to oscillate about a constant value in the pin-fin region, whereas SCS tends to oscillate about a line with a negative slope. For both test problems, the variation of SCS is not more complicated than Nu, but SCS yields the local heat flux without need for Tb, a parameter that is hard to define and measure for complicated problems.

Experimental Investigation of the Effect of Synthetic Jets in Minichannels With Subcooled Boiling Flow

2013 ◽  
Author(s):  
David M. Sykes ◽  
Andrew L. Carpenter ◽  
Gregory S. Cole
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Local Heat Transfer ◽  
Local Heat ◽  
High Heat ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Heat Transfer Augmentation

Microchannels and minichannels have been shown to have many potential applications for cooling high-heat-flux electronics over the past 3 decades. Synthetic jets can enhance minichannel performance by adding net momentum flux into a stream without adding mass flux. These jets are produced because of different flow patterns that emerge during the induction and expulsion stroke of a diaphragm, and when incorporated into minichannels can disrupt boundary layers and impinge on the far wall, leading to high heat transfer coefficients. Many researchers have examined the effects of synthetic jets in microchannels and minichannels with single-phase flows. The use of synthetic jets has been shown to augment local heat transfer coefficients by 2–3 times the value of steady flow conditions. In this investigation, local heat transfer coefficients and pressure loss in various operating regimes were experimentally measured. Experiments were conducted with a minichannel array containing embedded thermocouples to directly measure local wall temperatures. The experimental range extends from transitional to turbulent flows. Local wall temperature measurements indicate that increases of heat transfer coefficient of over 20% can occur directly below the synthetic jet with low exit qualities. In this study, the heat transfer augmentation by using synthetic jets was dictated by the momentum ratio of the synthetic jet to the bulk fluid flow. As local quality was increased, the heat transfer augmentation dropped from 23% to 10%. Surface tension variations had a large effect on the Nusselt number, while variations in inertial forces had a small effect on Nusselt number in this operating region.

Inverse Determination of the Local Heat Transfer Coefficients for Nucleate Boiling on a Horizontal Cylinder

2003 ◽  
Vol 125 (6) ◽  
pp. 1087-1095 ◽  
Author(s):  
H. Louahlia-Gualous ◽  
P. K. Panday ◽  
E. A. Artioukhine
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Pool Boiling ◽  
Gradient Methods ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Nucleate Boiling ◽  
Nucleate Pool Boiling ◽  
Transfer Coefficients ◽  
Iterative Regularization

This article treats the local heat transfer for nucleate pool boiling around the cylinder using the inverse heat conduction analysis. The physical model considers a half section of a cylinder with unknown surface temperature and heat flux density. The iterative regularization and the conjugate gradient methods are used for solving the inverse analysis. The local Nusselt number profiles for nucleate pool boiling are presented and analyzed for different electric heat. The mean Nusselt number estimated by IHCP is closed with the measured values. The results of IHCP are compared to those of Cornewell and Houston (1994), Stephan and Abdelsalam (1980) and Memory et al. (1995). The influence of the error of the measured temperatures and the error in placement of the thermocouples are studied.

Effect of Periodicity of Non-Uniform Sinusoidal Side Heating on Natural Convection in an Anisotropic Porous-Enclosure

2011 ◽  
Vol 110-116 ◽  
pp. 1613-1618 ◽  
Author(s):  
S. Kapoor ◽  
P. Bera
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Stream Function ◽  
Numerical Study ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Porous Enclosure

A comprehensive numerical study on the natural convection in a hydrodynamically anisotropic as well as isotropic porous enclosure is presented, flow is induced by non uniform sinusoidal heating of the right wall of the enclosure. The principal directions of the permeability tensor has been taken oblique to the gravity vector. The spectral Element method has been adopted to solve numerically the governing differential equations by using the vorticity-stream-function approach. The results are presented in terms of stream function, temperature profile and Nusselt number. The result show that the maximum heat transfer takes place at y = 1.5 when N is odd.. Also, increasing media permeability, by changing K* = 1 to K* = 0.2, increases heat transfer rate at below and above right corner of the enclosure. Furthermore, for the all values of N, profiles of local Nusselt number (Nuy) in isotropic as well as anisotropic media are similar, but for even values of N differ slightly at N = 2.. In particular the present analysis shows that, different periodicity (N) of temperature boundary condition has the significant effect on the flow pattern and consequently on the local heat transfer phenomena.

Investigation of Buoyancy Effects in Asymmetrically Heated Near-Critical Flows of Carbon Dioxide in Horizontal Microchannels Using Infrared Thermography

2021 ◽  
Author(s):  
Lindsey V. Randle ◽  
Brian M. Fronk
Keyword(s):  
Heat Transfer ◽  
Carbon Dioxide ◽  
Heat Flux ◽  
Infrared Thermography ◽  
Test Section ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Parallel Microchannels

Abstract In this study, we use infrared thermography to calculate local heat transfer coefficients of top and bottom heated flows of near-critical carbon dioxide in an array of parallel microchannels. These data are used to evaluate the relative importance of buoyancy for different flow arrangements. A Joule heated thin wall made of Inconel 718 applies a uniform heat flux either above or below the horizontal flow. A Torlon PAI test section consists of three parallel microchannels with a hydraulic diameter of 923 μm. The reduced inlet temperature (TR = 1.006) and reduced pressure (PR = 1.03) are held constant. For each heater orientation, the mass flux (520 kgm−2s−2 ≤ G ≤ 800 kgm−2s−2) and heat flux (4.7 Wcm−2 ≤ q″ ≤ 11.1 Wcm−2) are varied. A 2D resistance network analysis method calculates the bulk temperatures and heat transfer coefficients. In this analysis, we divide the test section into approximately 250 segments along the stream-wise direction. We then calculate the bulk temperatures using the enthalpy from the upstream segment, the heat flux in a segment, and the pressure. To isolate the effect of buoyancy, we screen the data to omit conditions where flow acceleration may be important or where relaminarization may occur. In the developed region of the channel, there was a 10 to 15 percent reduction of the local heat transfer coefficients for the upward heating mode compared to downward heating with the same mass and heat fluxes. Thus buoyancy effects should be considered when developing correlations for these types of flow.

Heat-Flux Measurements for a Realistic Cooling Hole Pattern With Multiple Flow Conditions

2013 ◽  
Author(s):  
Jeremy B. Nickol ◽  
Randall M. Mathison ◽  
Michael G. Dunn
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Local Heat Transfer ◽  
Axial Direction ◽  
Local Heat ◽  
Data Sets ◽  
Adiabatic Wall ◽  
Adiabatic Cooling ◽  
Cooling Hole ◽  
Flux Measurements

Predicting cooling flow migration and its impact on surface heat flux for a turbine operating at design-corrected conditions is a challenging task. While recent data sets have provided a baseline for comparison, they have also raised many questions about comparison methods and the proper implementation of boundary conditions. Simplified experiments are helpful for bridging the gap between the experimental and computational worlds to develop the best procedures for generating predictions and correctly comparing them to experiments. To this end, a flat-plate configuration has been developed that replicates the cooling hole pattern of the pressure side of a high-pressure turbine blade. The heat transfer for this configuration is investigated for a range of flow properties of current interest to the industry using a medium-duration blowdown facility. Heat-flux measurements are obtained using double-sided Kapton heat-flux gauges arrayed in two rows in the axial direction along the centerline of the hole pattern. Gauges are located upstream of the holes, in between rows of holes, and extending far downstream of the last row of holes. New parameters are proposed for analyzing the data including a corrected Stanton number and the length-corrected heat flux reduction parameter. These parameters are used for exploring the influence of Reynolds number and blowing ratio on local heat transfer. In addition, the temperatures of the main flow and the test section walls were varied to determine the effect of cooling on the local adiabatic wall temperature and to enable comparisons using the adiabatic cooling effectiveness.

scholarly journals Analysis of Backward and Forward Effects on a Grooved Co-axial Heat Exchanger by Response Surface Methodology

2019 ◽  
Vol 111 ◽  
pp. 01092
Author(s):  
Şahin GÜNGÖR ◽  
Levent AYDIN ◽  
Umut CEYHAN ◽  
Büşra KAYA ◽  
Ziya Haktan KARADENİZ
Keyword(s):  
Heat Transfer ◽  
Response Surface Methodology ◽  
Response Surface ◽  
Flow Direction ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heated Wall ◽  
Transfer Coefficients ◽  
Pipe Model ◽  
Energy Equations

In the literature, it is proved that grooved pipe models are thermally more efficient than the smooth pipe model. Different than the previous studies in which the groove dimensions are constant along the pipe, we study the effect of groove radius and the gap between adjacent grooves on the local heat transfer coefficients using computational fluid dynamics software. The grooved section consists of three sub-sections to see the effects of groove dimension in stream-wise flow direction. We vary the radius of circular grooves parametrically in each section to optimize the local groove radius throughout the pipe. We couple the fluid flow (1200<Re<24000) with energy equations, and the grooved sections are set as heated wall at constant temperature of 350 K. The optimal mesh has been selected by performing mesh independence study and finer mesh has been used in heated wall section. The radii of grooves are varied from 2 to 6 mm with an increment of 0.2 mm considering the manufacturability of the pipe, to do so we use the design of experiments (DOE). All DOE tools in ANSYS software are examined and compared with full factorial results. After DOE process, local heat transfer coefficient values of all groove parts are examined by response surface methodology (RSM).

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