Greater Heat Transfer Research Articles

Conjugate Heat Transfer for Mixed Convection and Maxwell Fluid on a Stagnation Point

A steady two-dimensional conjugate heat transfer with Ohmic dissipation mixed convection of an incompressible Maxwell fluid on a stagnation point has been studied. The whole system is an extrusion forming process. The governing equations have been solved by similarity transformation method and finite-difference method. The above two methods have been used to analyze present problem which is a different way to analysis the similar problems. The numerical solutions of the flow velocity distributions, temperature profiles, the wall unknown values f′′(0) and θ′(0) for calculating the heat transfer of the similar boundary layer flow had been carried out as function of the viscoelastic number, Prandtl number, buoyancy parameters, etc. The effects of those parameters have been discussed in detail. The results have been shown that it should produce greater heat transfer effect with larger values of free convection parameter, Prandtl number and conduction–convection number. On the other hand, the magnetic parameter or Eckert number will reduce the heat transfer effect.

Constructal Underground Designs for Ground-Coupled Heat Pumps

In this paper, we review the main advances made by our research group on the heat transfer performance of complex flow architectures embedded in a conducting solid. The immediate applications of this work include the design of ground-coupled heat pumps, seasonal thermal energy storage systems, and district heating and cooling systems. Various configurations are considered: U-shaped ducts with varying spacing between the parallel portions of the U, serpentines with three elbows, and trees with T-shaped and Y-shaped bifurcations. In each case, the volume ratio of fluid to soil is fixed. We found the critical geometric features that allow the heat transfer density of the stream-solid configuration to be the highest. In the case of U-tubes and serpentines, the best spacing between parallel portions is discovered, whereas the vascular designs morph into bifurcations and angles of connection that provide progressively greater heat transfer rate per unit volume. We show that the flow of heat into or out of a solid volume must have an S-shaped history curve that is entirely deterministic. This constructal-design principle unites a wide variety of previously disconnected S-curve phenomena (ground heat storage and retrieval, population growth, cancer, chemical reactions, contaminants, languages, news, information, innovations, technologies, economic activity).

Prediction of Swirl Flow and Heat Transfer through Square Duct with Twisted Tape Insert

Numerical predictions of characteristics of turbulent flows through a square duct (30 30 mm) with twisted tape inserts and with twisted tape inserts plus interrupted ribs are conducted to investigate regionally averaged heat transfer and friction factors by using CFX 11.0 commercial code. The validity of the numerical results is confirmed by measurement. Reynolds numbers are varied between 8,900 and 29,000. A rib height-to-channel hydraulic diameter (e/Dh) of 0.067 and a length-to hydraulic diameter (L/Dh) of 30 are considered. The square ribs are arranged to follow the trace of the twisted tape and along the flow direction defined as axial interrupted ribs. The twisted tape is 0.1 mm thick carbon steel sheet with diameter of 28mm, length of 900mm and 2.5 turns. Each wall of the square channel is composed of the isolated aluminum section. The present study demonstrates that the twisted tape with interrupted ribs provides a greater overall heat transfer performance over the twisted tape with no ribs in the square duct.

Effect of Prandtl Number on Free Convection in a Solar Collector Filled with Nanofluid

Numerical study of the influence of Prandtl number on free convection flow phenomena in a solar collector having glass cover plate and sinusoidal absorber is done. The working fluid is water-Al2O3 nanofluid. The cover plate has initially constant temperature Th, while bottom absorber is at temperature Tc, with Th > Tc. The remaining walls are considered adiabatic. By Penalty Finite Element Method the governing differential equations with boundary conditions are solved. The effect of the Prandtl number on the flow pattern and heat transfer has been depicted. Comprehensive average Nusselt number, average temperature and mean velocity inside the collector are presented as a function of the governing parameter mentioned above. The highest Pr causes the greatest heat transfer. The enhancing performance of heat transfer rate is more effective for the water-Al2O3 nanofluid than the base fluid.

Multimedia physical feature for unsteady MHD mixed convection viscoelastic fluid over a vertical stretching sheet with viscous dissipation

A multimedia feature for unsteady magnetic hydrodynamic (MHD) flow of two-dimensional mixed convection flow of an incompressible viscoelastic fluid over a vertical stretching sheet with viscous dissipation was studied. Similar transformation and an implicit finite-difference method were used to analyze the present problem. The numerical solutions of the flow velocity distributions, temperature profiles, the wall unknown values of f''(0) and -for calculating the heat transfer of the similar boundary-layer flow were carried out as functions of the unsteadiness parameter (S), the Eckert number (Ec), the Prandtl number (Pr), the magnetic parameter (M), the buoyancy parameter (G), the viscoelastic parameter (α), the space-dependent parameter (A) and temperature-dependent parameter (B) for heat source/sink. The effects of these parameters were also discussed. The results showed that greater heat transfer effect was produced with a larger S and α. On the other hand, parameters G, Pr, Ec, A and B reduced the heat transfer effect at some specific conditions. Key words: Mixed convection, viscous dissipation, MHD, multimedia feature, viscoelastic, heat transfer, unsteady flow, vertical stretching sheet.

An Experimental Study on the Effects of Nozzle and Surface Geometry in FC-72 Jet Cooling

ABSTRACTIn this study, a spray cooling device for electronic components was investigated. Dielectric fuid (FC-72) was sprayed at 50°C through five nozzles (4.243mm spacing). The nozzles are of diameters 0.17, 0.23 or 0.41mm. Volume flow rate varied from 24.5 to 99.1ml/min. Two grooved surfaces and a smooth surface were tested, and the heated area was 12 × 12mm2. The larger nozzles yielded greater heat transfer coefficients at high heat fluxes (300 ∼ 600kW/m2). However, smaller nozzles result in greater dry-out heat fluxes and greater heat transfer coefficients at heat flux < 300kW/m2. The C4 surface, having parallel grooves of 0.4mm depth, improved the spray cooling performance by up to 80% as compared with the smooth suface. Its thermal resistance is 0.11 ∼ 0.12K/W at 99.1ml/min flow rate, in the range of 85 ∼ 130W heat input. A new correlation of spray cooling, accounting for the contributions of nucleate boiling and spray convection, is proposed. For data of FC-72 in the range of Re = 856 ∼ 6188, Bo = 0.19 ∼ 5.70, We = 25.2 ∼ 3541.3, the predicted h-values agree with experimental data of the smooth surface within ±25%.

Microfibrous entrapment of Ni/Al2O3 for dry reforming of methane: a demonstration on enhancement of carbon resistance and conversion

Microfibrous entrapment technology in process intensification was demonstrated in the dry reforming of methane. The 8 μm-copper microfibrous entrapped Ni/Al2O3 (Cu-MFE-Ni/AlO) catalyst, with excellent permeability, provided great heat transfer enhancement, thereby leading to significant improvement of carbon resistance by a thermodynamic route along with visible conversion promotion. For instance, at a temperature of 1073 K, 4-fold reduction of average carbon deposition rate was achieved in the Cu-MFE-Ni/AlO composite bed compared to the packed bed of Ni/AlO.

Unsteady mixed convection visco-elastic flow and heat transfer in a thin film flow over a porous stretching sheet with internal heat generation

In this study, a two-dimensional unsteady mixed convection flow of an incompressible visco-elastic fluid over a porous thermal forming thin film stretching sheet with internal heat generation has been studied. Two novel items x and are presented for free convection in porous space important parameters and an internal heat generation phenomena, respectively. The similarity transformation and an implicit finite-difference method have been used to analyze the present problem. The numerical solutions of the flow velocity distributions, temperature profiles, the wall unknown values of f''(0) and for calculating the heat transfer of the similar boundary-layer flow are carried out as functions of the unsteadiness parameter (S), the Prandtl number (Pr), the visco-elastic parameter (α), the porous parameter (), the space-dependent parameter (A) and temperature-dependent parameter (B) for heat source/sink and the free convection parameter (G). The effects of these parameters have also been discussed. The results show that it will produce greater heat transfer effect with a larger Pr, G and α, but S, A and B will reduce heat transfer effect. Key words: Visco-elastic, thin film flow, porous medium, finite-difference method, heat transfer, unsteady stretching sheet, mixed convection, internal heat generation.

Transient cooling of a room with a chilled ceiling

Using passive systems or solar energy for space cooling is highly conditional upon the heat transfer potential between the building components and the interior space. This paper presents a simplified theoretical model that describes the evolution of temperature in a room that is cooled by a chilled ceiling in the presence of a heat source on the floor. Using arguments of dynamic similarity, the theoretical model is confirmed with scale-model experiments run in a rectangular tank. The scale model uses a metallic heat exchanger to simulate the chilled ceiling and an electric heater as the heat source. The greatest heat transfer from the fluid towards the ceiling occurs immediately with the simultaneous activation of the heater and the chilled ceiling. As time elapses and the heat transfer decreases, the room cools more slowly and its temperature tends asymptotically towards equilibrium. This is achieved when the heat loss through the ceiling equals the heat supply from the heat source. The theoretical model can predict when a chilled ceiling, acting as transient a heat sink, will achieve a comfortable temperature in a room with an internal heat source.

Finite Element Simulation of Nickel-Based Alloy Cutting Process Based on DEFORM- 3D

To analysis cutting mechanism and heat transfer coefficient of Ni-based superalloy IN718, this paper introduced a three-dimensional finite element model for cutting process based on DEFORM-3D V6.1, and cutting force Fy and cutting temperature T under different feed rate f , cutting speed v and back engagement ap were obtained. The temperature variation during cutting process under different cooling method was analyzed. The simulation results indicate that the influence of back engagement ap on the cutting force Fy is the greatest, while that of cutting speed v is minor; and for cutting temperature T, influence of cutting speed v is the greatest, while that of back engagement ap is minor. Influence of heat transfer coefficient on highest temperature and average cutting force Fy is minor, but its influence on average temperature is quite obvious. Greater heat transfer coefficient is less average temperature.

Numerical Simulation of Characteristics of Cross-Flow Heat Transfer in Pulsating Flow

In order to investigate the characteristics of heat transfer in oscillating flow, the computational fluid dynamics method was employed to study the effects of pulsating flow on the heat transfer process in a cross-flow heat exchange pipe, and to analyze the underling mechanism which controls the improvement of heat transfer in pulsating flow through the distribution of temperature. Several pulsating frequencies (f=0, 5, 10, 50, 100, 150 Hz) and a wide range of pulsating amplitudes (inlet velocity u=2.0+Asin(2πft) m/s, A=0, 2, 5, 10, 15, 20 m/s) were explored to find out the best pulsating parameters for heat transfer. Results showed that pulsating flow with a low pulsating frequency (the magnitude of ~101 Hz) should be selected to obtain large heat transfer coefficient, and that pulsating flow with larger pulsating amplitude results in greater heat transfer coefficient. On the other hand, results revealed that only a limited length of the cross-flow exchange pipe was affected by the pulsating flow compared to the whole length, and that the affected length is longer with lower pulsating frequency and larger pulsating amplitude.

Combined Effects of Temperature and Reynolds Number on Heat Transfer Characteristics of a Cationic Surfactant Solution

The heat transfer characteristics of a dilute cationic surfactant solution, CTAC/NaSal aqueous solution, were experimentally investigated in a fully developed two-dimensional water channel flow at different temperatures ranging from 15°C to 60°C and at different mass concentrations, namely, 75, 100, and 200 ppm. The magnitudes of the maximum achievable heat transfer reduction (HTR) at the above three different surfactant concentrations are all about 90%. The cationic surfactant solution showed a great heat transfer reduction phenomenon, which was greatly affected by concentration, temperature, and Reynolds number. It was supposed that temperature and shear stress are two kinds of energy applied on the surfactant microstructure, which can be helpful to the surfactant network formation or dissociation depending on their values.

Second-Law Analysis to Improve the Energy Efficiency of Screw Liquid Chillers

This work applies the second-law analysis of thermodynamics to quantify the exergy destruction of the components of screw liquid chiller, and to identify the potential for each component to contribute to improve the overall energy efficiency of the system. Three screw liquid chiller units were built to demonstrate the feasibility of the model presented herein. Unit A was a 100 RT water-cooled screw liquid chiller. Unit B was modified from Unit A by switching the old condenser for a new one with a greater heat transfer, and Unit C was modified from Unit B by exchanging the compressor for a more efficient one. The results indicate that the compressor has the largest potential to improve energy efficiency, followed in order by the condenser, and then the evaporator. The second law analysis may help engineers to focus on the components with higher exergy destruction and quantify the extent to which modifying such components can influence, favorably or unfavorably, the performance of other components of the screw liquid chiller.

Two-phase flow heat transfer of R-134a in microtubes

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (Di = 430 μm) was 47.0% greater than that for the tube B (Di = 792 μm) at G = 370 kg/m2·s and q″ = 20 kW·m2. A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.

Nanoreactors for Aqueous RAFT-Mediated Polymerizations

Nanoreactors provide the ideal setting where selected chemical reactions can take place with high efficiency in controlled environments. Much attention has been focused on carrying out organic reactions in surfactant-based micelles in water.(1) This methodology can provide significant advantages over solution reactions due to compartmentalization of reactants or catalysts, greater heat transfer through the aqueous environment, controlled rates of reaction, better accommodation for potentially explosive or runaway exothermic reactions, and the environmentally friendly nature of the aqueous reaction medium.(2, 3)The concept of nanoreactors is not new to free-radical polymerizations, in which surfactant stabilized micelles are commonly used.(4) However, these micelle-type nanoreactors (termed ab initio emulsion polymerizations) failed to produce well-controlled molecular weight distributions (MWDs) and particle size distributions (PSDs) in reversible addition−fragmentation chain transfer (RAFT) controlled/“living” radical polymerizations (CRP) due to transportation problems of monomer or other compounds (e.g., RAFT agent) from monomer droplet reservoirs to the growing particles.(3) Miniemulsions have solved the transportation problem as each droplet is an isolated bulk nanoreactor, in which all components are located within the droplet.(5) The drawbacks of miniemulsions are (a) the difficulty in isolating pure polymer product due to the high levels of hexadecane (to reduce Oswald ripening) and surfactant and (b) the PSDs are broad with little or no control over the particle size. Other methods of growing amphiphilic diblock copolymers in situ led to poor control of the chain length polydispersity (>1.4, while below 1.2 is considered “good control”), broad PSDs,(6) and very long polymerization times.(7)In our quest to find better alternative water-based methods to carry out RAFT-mediated polymerizations, specially designed nanoreactors have provided the solution to produce narrow and controlled molecular weight and particle size distributions. The nanoreactors used in this work were designed to (a) encapsulate the hydrophobic or water insoluble monomer, (b) selectivity bind the RAFT agent directly in the nanoreactor to avoid transportation problems,(8) (c) provide steric colloidal stabilization of the nanoparticles, (d) produce monodisperse nanoparticles, and (e) be easily removed to obtain the pure synthesized polymer (see Scheme 1). We created the nanoreactor from a diblock copolymer of poly(N-isopropylacrylamide-b-dimethylacrylamide) or P(DMA49-b-NIPAM106), which for the PNIPAM block(9) is hydrophilic below its lower critical solution temperature (LCST 36 °C)(10) and water-soluble and above its LCST forms self-stabilized, through the hydrophilic PDMA block, monodisperse polymer particles in the nanosize range (22.4 nm). The RAFT agent, a low molecular weight PNIPAM18−SC(═S)SC4H9 (MacroCTA), was designed to be highly compatible with the nanoreactor and when self-assembled with the diblock above the LCST of PNIPAM gave a hydrodynamic diameter of 31.5 nm (see Table S1 in Supporting Information). The great advantage of our methodology is that water, P(DMA49-b-NIPAM106), PNIPAM18−SC(═S)SC4H9, styrene monomer, and water-soluble initiator can be added in one pot and heated above the LCST to form the nanoreactors

Effects of fire intensity on survival and recovery of soil microarthropods after a clearcut burning

We studied responses of soil microarthropods to different burning intensities at a clearcut that was burnt in May 2002. Fire intensity was manipulated by adding or removing logging residues as fuel from the experimental plots. Samples were taken 1 week before and 1 week after burning as well as during autumn of the same year. Samples were taken in the 2 following years to estimate long-term recovery. No difference in humus combustion could be detected between burning intensities, but most microarthropod species showed lower abundances in the hard-burnt than in the light-burnt plots immediately after fire. Surface-living species also declined in light-burnt plots, whereas soil-living species were particularly affected in hard-burnt plots. This is probably explained by greater heat transfer into the hard-burnt soil. Total abundances of Oribatida and Protura remained low for several years in the burnt plots, whereas abundances of Collembola and Mesostigmata recovered within 1 year, which indicates that at least these groups had enough habitat space and food resources after fire. The study indicates that fire severity (depth of burn) is more decisive than fire intensity (heat release) for the long-term recovery of soil fauna, whereas fire intensity determines the acute survival of animals.

Effects of Nonuniformly Heated Wall(S) on a Natural-Convection Flow in a Square Cavity Filled With a Porous Medium

The influence of uniform and nonuniform heating of wall(s) on natural-convection flow in a square cavity filled with a porous matrix has been studied numerically by using the penalty finite-element method with biquadratic rectangular elements. In the present investigation, the left vertical wall and the bottom wall are uniformly and nonuniformly heated, while the right vertical wall is maintained at constant cold temperature and the top wall is well insulated. The Darcy-Forchheimer model is used to simulate the momentum transfer in the porous medium. The present numerical approach yields consistent performance over the range of parameters (Rayleigh number Ra, 103 ≤ Ra ≤ 106, Darcy number Da, 10−5 ≤ Da ≤ 10−3, and Prandtl number Pr, 0.71 ≤ Pr ≤ 10) in order to obtain the solutions in terms of stream functions, temperature profiles, and Nusselt numbers. Nonuniform heating of the bottom wall produces greater heat transfer rate at the center of the bottom wall than the uniform heating case for all Rayleigh numbers, but average Nusselt number shows overall lower heat transfer rate for the nonuniform heating case. Critical Rayleigh numbers for conduction-dominant heat transfer cases have been obtained. For convection-dominated regimes the power-law correlations between average Nusselt number and Rayleigh numbers are presented.

Non-uniformity of heat transfer in a shuttle kiln

A numerical investigation of the flow and heat transfer characteristics in a shuttle kiln is performed using the CFD software FLUENT with non-conformal grids and the standard κ-e turbulence model. The integral and local heat transfer non-uniformities of stacks are evaluated, and furthermore, the formation mechanism of such non-uniformities is analyzed. The results show much greater heat transfer intensity among the peripheral stacks than that among the internal ones, which contributes to the integral non-uniformity. The longitudinal heat transfer non-uniformity is primarily caused by the layer-like nozzle jets and bottom outlet, which form three layer-style surrounding streams converging to outlet. The periodical arrangement of stacks together with the strong impingement of jets, leads to the peripheral heat transfer non-uniformity. Based on the comprehensive understanding of flow and heat transfer, innovative efforts are made to improve heat transfer performance by adjusting flowrate distribution among nozzles

Effects of substrate surface conditions on heat transfer and shell morphology in the solidification of a copper alloy

The effect of substrate surface conditions on the heat transfer and morphology of solidifying shells of a copper alloy has been characterized by a series of dip tests. Results showed that in the early stages of solidification, rough and grooved surface conditions provide a greater heat transfer than polished ones. The shells solidified on rough and grooved surfaces had significant localized variations in thickness, indicating that heat transfer at the substrate-metal interface was not uniform. The implication of these results on strip casting is discussed.

Heat transfer practice with organic media

This new edition represents not only the most recent level of development using organic media, but also the status of design regulations which affect the design, planning, construction, and operation of plants of this type. It is the most up-to-datetreatment of this technique which permits transferring and regulating heat up to and beyond 300°C almost without increase in pressure. The guide covers in great depth heat transfer media, design of heat transfer plants, fundamentals of heat and flow, heaters, plant components, heat consumers, measuring and control techniques, installation specifications, operation of heat transfer plants, and examples of actual heat transfer plants. A voluminous Appendix gives detailed properties of organic heat carriers, such as: organic liquids having a mineral oil or a synthetic base, organic media that can operate in either a liquid or vapor state, and special oils. 664 pages, © 1997