Showing papers on "Heat transfer coefficient published in 2001"

Principles of Heat Transfer

Abstract: Preface. Guide to Instructors and Students. Acknowledgments. 1. Introduction and Preliminaries. 1.1. Applications and History. 1.2. Units and Normalization (Scaling). 1.3. Thermal Systems. 1.4. Heat Flux Vector q. 1.5. Heat Transfer Medium. 1.6. Conservation of Energy. 1.7. Conservation of Mass, Species, and Momentum. 1.8. Scope. 1.9. Summary. 1.10. References. 1.11. Problems. 2. Energy Equation. 2.1. Nonuniform Temperature Distribution: Differential--Volume Energy Equation. 2.2. Uniform Temperature in One or More Directions: Energy Equation for Volumes with One or More Finite Lengths. 2.3. Energy Conversion Mechanisms. 2.4. Bounding--Surface and Far--Field Thermal Conditions. 2.5. Heat Transfer Analysis. 2.6. Summary. 2.7. References. 2.8. Problems. 3. Conduction. 3.1. Microscale Heat Storage and Specific Heat Capacity c p . 3.2. Microscale Conduction Heat Carriers and Thermal Conductivity k. 3.3. Steady--State Conduction. 3.4 Transient Conduction. 3.5 Distributed--Capacitance (Nonuniform Temperature) Transient: T = T(x, t). 3.6. Lumped--Capacitance (Uniform Temperature) Transient: Internal--External Conduction Number N k 0.1, T = T(t). 3.7. Discretization of Medium into Finite--Small Volumes. 3.8. Conduction and Solid--Liquid Phase Change: Stefan Number Ste l . 3.9. Thermal Expansion and Thermal Stress. 3.10. Summary. 3.11. References. 3.12. Problems. 4. Radiation. 4.1. Microscale Heat Emission: Photon and Surface Thermal Radiation Emission. 4.2. Interaction of Irradiation and Surface. 4.3. Thermal Radiometry. 4.4. Enclosure Surface Radiation Heat Transfer Q r,I Among Gray, Diffuse, and Opaque Surfaces. 4.5. Prescribed Irradiation and Nongray Surfaces. 4.6. Inclusion of Substrate. 4.7. Summary. 4.8. References. 4.9. Problems. 5. Convection: Unbounded Fluid Streams. 5.1. One--Dimensional Conduction--Convection Energy Equation. 5.2. Parallel Conduction--Convection Resistance R k,u and Conduction--Convection Number N u = Pe L . 5.3. Evaporation Cooling of Gaseous Streams. 5.4. Combustion Heating of Gaseous Streams. 5.5. Joule Heating of Gaseous Streams. 5.6. Gas--Stream Volumetric Radiation. 5.7. Summary. 5.8. References. 5.9. Problems. 6. Convection: Semi--Bounded Fluid Streams. 6.1. Flow and Surface Characteristics. 6.2. Semi--In.nite Plate as a Simple Geometry. 6.3. Parallel, Turbulent Flow: Transition Reynolds Number Re L,t 6.4. Perpendicular Flows: Impinging Jets. 6.5. Thermobuoyant Flows. 6.6. Liquid--Gas Phase Change. 6.7. Summary of Nusselt Number Correlations. 6.8. Inclusion of Substrate. 6.9. Surface--Convection Evaporation Cooling. 6.10. Summary. 6.11. References. 6.12. Problems. 7. Convection: Bounded Fluid Streams. 7.1. Flow and Surface Characteristics. 7.2. Tube Flow and Heat Transfer. 7.3. Laminar and Turbulent Flows, Entrance Effect, Thermobuoyant Flows, and Phase Change. 7.4. Summary of Nusselt Number Correlations. 7.5. Inclusion of Bounding Solid. 7.6. Heat Exchange Between Two Bounded Streams. 7.7. Summary. 7.8. References. 7.9. Problems. 8. Heat Transfer in Thermal Systems. 8.1. Primary Thermal Functions. 8.2. Thermal Engineering Analysis. 8.3. Examples. 8.4. Summary. 8.5. References. 8.6. Problems. Nomenclature. Glossary. Answers to Problems. A. Some Thermodynamic Relations. A.1. Simple, Compressible Substance. A.2. Phase Change and Heat of Phase Change. A.3. Chemical Reaction and Heat of Reaction. A.4. References. B. Derivation of Differential--Volume Energy Equation. B.1. Total Energy Equation. B.2. Mechanical Energy Equation. B.3. Thermal Energy Equation. B.4. Thermal Energy Equation: Enthalpy Formulation. B.5. Thermal Energy Equation: Temperature Formulation. B.6. Conservation Equations in Cartesian and Cylindrical Coordinates. B.7. Bounding Surface Energy Equation with Phase Change. B.8. References. C. Tables of Thermochemical and Thermophysical Properties 899 C.1. Tables. Unit Conversion, Universal Constants, Dimensionless Numbers, Energy Conversion Relations, and Geometrical Relations. Periodic Table and Phase Transitions. Atmospheric Thermophysical Properties. Electrical and Acoustic Properties. Thermal Conductivity. Thermophysical Properties of Solids. Surface--Radiation Properties. Mass Transfer and Thermochemical Properties of Gaseous Fuels. Thermophysical Properties of Fluids. Liquid--Gas Surface Tension. Saturated Liquid--Vapor Properties. C.2. References. D. SOlver for Principles of Heat Transfer (SOPHT). D.1. Objective. D.2. SOPHT. List of Key Charts, Figures, and Tables. Subject Index.

3ω method for specific heat and thermal conductivity measurements

Abstract: We present a 3ω method for simultaneously measuring the specific heat and thermal conductivity of a rod- or filament-like specimen using a way similar to a four-probe resistance measurement. The specimen in this method needs to be electrically conductive and with a temperature-dependent resistance, for acting both as a heater to create a temperature fluctuation and as a sensor to measure its thermal response. With this method, we have successfully measured the specific heat and thermal conductivity of platinum wire specimens at cryogenic temperatures, and measured those thermal quantities of tiny carbon nanotube bundles some of which are only ∼10−9 g in mass.

Experimental investigation on condensation heat transfer and pressure drop of new HFC refrigerants (R134a, R125, R32, R410A, R236ea) in a horizontal smooth tube

Abstract: This paper reports experimental heat transfer coefficients and pressure drops measured during condensation inside a smooth tube when operating with pure HFC refrigerants (R134a, R125, R236ea, R32) and the nearly azeotropic HFC refrigerant blend R410A. Data taken when condensing HCFC-22 are also reported for reference. The experimental runs are carried out at a saturation temperature ranging between 30 and 50°C, and mass velocities varying from 100 to 750 kg/(m2 s), over the vapour quality range 0.15–0.85. The effects of vapour quality, mass velocity, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. A predictive study of the condensation flow patterns occurring during the tests is also presented. Finally comparisons with predictions from the model by Kosky and Staub (Kosky PG, Staub FW. Local condensing heat transfer coefficients in the annular flow regime. AIChE J 1971;17:1037) are reported for all the data sets.

Heat transfer over an exponentially stretching continuous surface with suction

Abstract: Similary solutions of the laminar boundary layer equations describing heat and flow in a quiescent fluid driven by an exponentially stretching surface subject to suction are examined numerically. The direction and amount of heat flow were found to be dependent on the magnitude of " γ " (parameter of temperature) for the same Prandtl number. Nusselt number increases with increasing " γ " and the Prandtl number. The effect of decreasing suction parameter is found to be significant particularly for the Prandtl number.

Microstructure devices for applications in thermal and chemical process engineering

Abstract: Metallic microstructure devices have been manufactured and tested for applications in technical applications in thermal and chemical process engineering and in the laboratory. Manufacturing has been performed by micromachining of metal foils and diffusion bonding of a laminated foil stack, followed by welding of the microstructured core into a housing. Crossflow and counterflow micro heat exchangers with microchannel or microcolumn structures have been developed for throughputs up to 7 t water/h with a heat transmission power up to 200 kW. Overall heat transfer coefficients up to 54,000 W/m2 K have been determined with water as test fluid. Electrically driven microstructured heaters have been developed for fast and precise heating of sensitive liquids or gases. The heater has been used as an evaporator as well. Static micromixers have been developed for fast and complete mixing of reactands. They can be utilized with parallel reactions, for example, to decrease the yield of undesired by-products. For hete...

Radiative heat transfer between nanostructures

Abstract: We use a general theory of the fluctuating electromagnetic field and a generalized Kirchhoff's law (Ref. 8) to calculate the heat transfer between macroscopic and nanoscale bodies of arbitrary shape, dispersive, and absorptive dielectric properties. We study the heat transfer between: (a) two parallel semi-infinite bodies, (b) a semi-infinite body and a spherical body, and (c) two spherical bodies. We consider the dependence of the heat transfer on the temperature T, the shape and the separation d, and discuss the role of nonlocal and retardation effects. We find that for low-resistivity material the heat transfer is dominated by retardation effects even for the very short separations.

Two-phase heat transfer to a refrigerant in a 1 mm diameter tube

Abstract: A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m2 s and heat flux range of 10–1150 kW/m2. In this paper the boiling map for a mass velocity of 510 kg/m2 s and heat flux of 18–72 kW/m2 is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.

Boiling Heat Transfer and Bubble Dynamics in Microgravity

Abstract: This article presents results for pool boiling heat transfer under microgravity conditions that the author and his team have gained in a succession of experiments during the past two decades. The objective of the research work was to provide answers to the following questions: Is boiling an appropriate mechanism of heat transfer for space application? How do heat transfer and bubble dynamics behave without the influence of buoyancy, or more general, without the influence of external forces? Is microgravity a useful environment for investigating the complex mechanisms of boiling with the aim of gaining a better physical understanding? Various carrier systems that allow simulation of a microgravity environment could be used, such as drop towers, parabolic trajectories with NASA’s aircraft KC-135, ballistic rockets such as TEXUS, and, more recently, three Space Shuttle missions. As far as the possibilities of the respective missions allowed, a systematic research program was followed that was continuously adjusted to actual new parameters. After a general survey concerning the importance of boiling for technical applications, an introduction is given especially for those individuals not closely familiar with the fields of microgravity and boiling. Surprising results have been obtained: not only that saturated pool boiling can be maintained in a microgravity environment, but also that at small heater surfaces and lower values of heat fluxes, even higher heat transfer coefficients have been attained than under terrestrial conditions. The bubble departure can be attributed to surface tension effects, to “bubble ripening” and coalescence processes. Under subcooled conditions only, thermocapillary flow was observed that transports the heat from the bubble interface into the bulk liquid, but does not enhance the heat transfer compared with boiling at saturated conditions. Direct electrical heated plane surfaces lead to a slow extension of dry spots to dry areas below bubbles, the increasing surface temperature suggesting transition to film boiling. The critical heat flux in microgravity is lower than under earth conditions, but considerably higher than the hitherto accepted correlations predict when extrapolated to microgravity. The nearly identical heat transfer coefficients received for nucleate boiling under microgravity as well as terrestrial conditions, and for both saturated and subcooled fluid states, also suggest identical heat transfer mechanisms. These results lead to the conclusion that the primary heat transfer mechanism must be strongly related to the development of the microlayer during bubble growth. Secondary mechanisms are responsible for the transport of enthalpy in form of latent energy of the bubbles and hot liquid carried with them. Under terrestrial conditions, that mechanism is caused by external forces such as buoyancy; under microgravity conditions, the self dynamics of the bubbles and/or thermocapillary flow under subcooled conditions are responsible. The results demonstrate clearly that boiling can be applied as a heat transfer mechanism in a microgravity environment and that microgravity is a useful means to study the physics of boiling.

A converging slot-hole film-cooling geometry - Part 1: Low-speed flat-plate heat transfer and loss

Abstract: This paper presents experimental measurements of the performance of a new film cooling hole geometry - the Converging Slot-Hole or Console. This novel, patented geometry has been designed to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems. The physical principles embodied in the new hole design are described, and a typical example of the console geometry is presented. The cooling performance of a single row of consoles was compared experimentally with that of typical 35° cylindrical and fan-shaped holes and a slot, on a large-scale, flat-plate model at engine representative Reynolds numbers in a low speed tunnel with ambient temperature main flow. The hole throat area per unit width is matched for all four hole geometries. By independently varying the temperature of the heated coolant and the heat flux from an electrically heated, thermally insulated, constant heat flux surface, both the heat transfer coefficient and the adiabatic cooling effectiveness were deduced from digital photographs of the colour play of narrowband thermochromic liquid crystals on the model surface. A comparative measurement of the aerodynamic losses associated with each of the four film-cooling geometries was made by traversing the boundary layer at the downstream end of the flat plate. The promising heat transfer and aerodynamic performance of the console geometry have justified further experiments on an engine representative nozzle guide vane in a transonic annular cascade presented in Part 2 of this paper [1].

Condensation and evaporation heat transfer of R410A inside internally grooved horizontal tubes

Abstract: Heat transfer coefficient and pressure drop were measured for condensation and evaporation of R410A and HCFC22 inside internally grooved tubes. The experiments were performed for a conventional spiral groove tube of 8.01 mm o.d. and 7.30 mm mean i.d., and a herring-born groove tube of 8.00 mm o.d. and 7.24 mm mean i.d. To measure the local heat transfer coefficients and pressure drop, the test section was subdivided into four small sections having 2 m working length. The ranges of refrigerant mass flow density was from 200 to 340 kg/(m2 s) for both condensation and evaporation of R410A and HCFC22, and the vapour pressure was 2.41 MPa for condensation and 1.09 MPa for the evaporation of R410A. The obtained heat transfer data for R410A and HCFC22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of spiral one for condensation and are slightly larger than those of spiral one for the evaporation. The measured local pressure drop in both condensation and evaporation is well correlated with the empirical equation proposed by the authors.

Film-Cooled Turbine Endwall in a Transonic Flow Field: Part II—Heat Transfer and Film-Cooling Effectiveness

Abstract: Thermodynamic and aerodynamic measurements were carried out in a linear turbine cascade with transonic flow field. Heat transfer and adiabatic film-cooling effectiveness resulting from the interaction of the flow field and the ejected coolant at the endwall were measured and will be discussed in two parts. The investigations were performed in the Windtunnel for Straight Cascades (EGG) at the DLR, Goettingen. The film-cooled NGV endwall was operated at representative dimensionless engine conditions of Mach and Reynolds number Ma2is=1.0 and Re2=850,000 respectively.Part II of the paper discusses the thermodynamic measurements. Detailed temperature measurements were carried out on a turbine stator endwall. In order to determine the surface heat transfer and adiabatic film-cooling effectiveness, an infrared camera system in a rectilinear wind tunnel measured the endwall temperatures in the transonic flow field. The adiabatic film-cooling effectiveness was determined using the superposition method. Measurements were carried out to first, validate the assumptions of theory and second, analyse the error associated with the measurements. Effects of coolant ejection from a slot and three rows of holes were investigated and compared with each other. The influence of Mach number and blowing ratio on the heat transfer were further aspects of the investigation. Strong variations in heat transfer and film-cooling effectiveness due to the interaction of the coolant air and the secondary flow field were found. Based on the results of this investigation an improved cooling design will be investigated in a follow-up project.© 2001 ASME