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• dawid420

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• Capa límite
• Transferencia de calor
• Conduccion termica
• Conductividad térmica
• Flujo laminar

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CH1 What are the physical mechanisms associated with heat transfer by conduction, convection, and radiation? Conduction- no bulk or macroscopic motion Convection- random molecular motion, diffusion, bulk, or macroscopic motion Radiation- emitted of energy What is the driving potential of heat transfer? Temperature difference (high to low) What is the difference between a heat flux and a heat rate? Heat flux- q”, W/m^2, heat divided by an area Heat rate- q, W, Product of heat flux and area What is a temperature gradient, dt/dx, K/m, What is thermal conductivity? K, W/m*K, it is a transport property. What is Fourier’s Law? q”=-k(dt/dx) What is the difference between natural convection and forced convection? Natural- the flow is induced by buoyancy forces, which are due to density differences caused by temperature variations in the fluid. Forced- flow is caused by external, like a fan What conditions are necessary for the development of a hydrodynamic boundary layer? Hydrodynamic- the velocity varies from zero at the surface to a finite value  ambient associated with the flow, velocity varies to zero in a hydrodynamic layer. What conditions are necessary for the development of a thermal boundary layer? Thermal- Ts @ y=0 to T ambient in the outer flow, velocity varies. What is Newton’s Law of cooling? q”=h(Ts-T ambient) What role is played by the convection heat transfer coefficient in Newton’s law of cooling? It depends on conditions in the thermal boundary layer, which are influenced by the surface geometry, the nature of the fluid motion, and an assortment of fluid thermodynamics and transport properties. h[W/m^2*K] What effect does convection heat transfer from or to a surface have on the solid bounded by the surface? none What is predicted by Stefan-Boltzmann law, and what unit of temperature must be used with this law? The upper limit to the emissive power, the unit of temperature is absolute k, Eb=Ts^4 What is the emissivity, and what role does it play does it play in characterizing radiation transfer at a surface? Emissivity- a radiation property of the surface; this property provides a measure of how efficiently a surface emits energy relative to a blackbody or ideal radiator. What is Irradiation? Irradiation- the rate at which all such radiation is incident on a unit area of the surface. Units are W/m^2. What conditions are associated with use of the radiation heat transfer coefficient? Hr, depends strongly on temperature. Net radiation= q”rad=q/A=Eb(Ts)-G=(Ts^4- Tsurr^4) What is the inherent difference between application of conservation of energy over a time interval and at an instant of time? Time interval- all energy terms are measured in joules Instant of time- all energy terms are balanced and measured in joules/s What is thermal energy storage? How does it differ from thermal energy generation? What role do the terms play in a surface energy balance? Energy generated- Eg – at an instant. Energy storage- over a  CH2 In the general formulation of Fouriers Law what are the vector and scalar quantities? Why is there a minus sign on the right hand side of the equation? Fouriers law has scalar temperature field and heat flux vector The minus sign is necessary because heat is always transferred in the direction of decreasing temperature What is an isothermal surface? What can be said about the heat flux at any location on this surface? The direction of heat flow will always be normal to a surface of constant temperature, the direction of q” is normal to the cross sectional area What form does fourier’s law take for each of the orthogonal directions of Cartesian cylindrical and spherical coordinate systems? In each case what are the units of the temperature gradient? Can you write each equation from memory? Cartisian Cylinderical

Spherical An important property of matter is defined by Fourier’s law. What is it? What is its physical significance? What are its units? k, thermal conductivity, W/ m*K, w/o k all materials that have the same A, ΔT, Δx would technically be the same What is isotropic material? A material where thermal conductivity is independent of the coordinate direction. Why is the thermal conductivity of a solid generally larger than that of a liquid? Why is the thermal conductivity of a liquid generally larger than that of a gas? Thermal conductivity is greater in solid than liquid and liquid than gas because the heat fluxes are higher. Why is the thermal conductivity of an electrically conducting solid generally larger than that of a non conductor? Why are materials such as beryllium oxide, diamond, and silicon carbide exceptions to this rule? In pure metals, the electron contribution to conduction heat transfer dominates, while non-conductors and semiconductors, the phonon contribution is dominant because they have high thermal conductivity. Is the effective thermal conductivity of an insulation system a true manifestation of the efficacy with which heat is transferred through the system by conduction alone? No, heat is also transferred due to surface radiation and the nature of the volumetric fraction of the air or void space. Why does the thermal conductivity of a gas increase with increasing temperature? Why is it approximately independent of pressure? Thermal conductivity of as gas increase w/ increasing temperature because č increases with increasing temperature and decreasing molecular weight. Because ρ and λmfp are directly and inversely proportional to the gas pressure. What is the physical significance of the thermal diffusivity? How is it defined and what are its units? Thermal diffusivity, α, it measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy. m2/s For a chemically reacting medium, what kind of reaction provides a source of thermal energy (q .>0)? What kind of reaction provides a sink for thermal energy (q.0 a source if thermal is being generated in the material at the expense of some other energy form. If q*2 thin with close spacing what is the fin efficiency? What is its range of possible values? Under what conditions will the efficiency be large? ή = qf / qmax = qf / hA Φb values from 0-1 for a maximum qmax = qf CH4 What is an isotherm? What is a heat flow line? How are the two lines related geometrically? When local heat flux in the solid is a vector that is everywhere perpendicular to the lines of constant temperature. Lines that represent the directions of the heat flux vector. They are perpendicular to each other. What is an adiabatic? How is it related to a line symmetry? How is it intersected by an isotherm? A line where no heat can be conducted across. Adiabatic surfaces are heat flow lines. perpendicular What parameters characterize the effect of geometry on the relationship between the rate and the overall temperature difference for steady conduction in a 2-D system? How are these parameters related to the conduction resistance? The average temperature of a certain region. What is represented by the temperature of a nodal point, and how does the accuracy of a nodal temperature depend on prescription of the nodal between? The larger the grid the finer the mesh, the more accurate. CH 5 Under what conditions may the lumped capacitance method be used to predict the transient response of a solid to a change in its thermal environment? Bi < .1 What is the physical interpretation of the Biot Number? Ratio of internal and eternal thermal resistances Is the lumped capacitance method of analysis likely to be more applicable for a hot solid being cooled by forced convection in air or in water? By forced convection in air or natural convection in air? Air, Air Is the lumped capacitance method of analysis likely to be more applicable for cooling of a hot solid made of copper or aluminum? For silicon nitride or glass? Copper What parameters determine the time constant associated with the transient thermal response of a lumped capacitance solid? Is this response accelerated or decelerated by an increase in the convection coefficient? By an increase in the density or specific heat of the solid? ρcvhA accelerated decelerated CH 6 What is the difference between a local convection heat transfer coefficient and average coefficient? What are their units? Local at any distance x from the leading edge, average is the entire surface W/m62*K

What are the forms of Newton’s law of cooling for a heat flux and a heat rate q”=h∆T q’=hA∆T ∆T= Ts-Tω What are the velocity and thermal boundary layers? Under what conditions do they develop A thin layer in which velocity gradients and shear stresses are larger, a thing fluid laying in which temperature gradients are present Velocity develops when there is a fluid flow over a surface, thermal develops if the fluid is free stream and the surface temp differs What quantities change with location in a velocity boundary layer? A thermal boundary layer? Velocity boundary layer thickness δ surface shear stress τ Thermal boundary layer thickness δ magnitude dT/dy!y=0 decreases as x increases q” and h decrease when x increases Recognizing that convection heat transfer is strongly influenced by conditions associated with fluid flow over a surface, how is it that we may determine the convection heat flux by applying fouriers law to the fluid at the surface? h∆T=-kdT/dy!y=0 h=-k dT/dy!y=0/∆T ∆T = Ts-Tω Do we expect heat transfer to change with transition from a laminar to a turbulent boundary layer? How? Yes,, Increases What physical processes are represented by the terms of the x-momentum equation (6.16)? By the energy equation (6.17)? Represents he net outflow of mass in the x & y directions, the sum of which must be zero for SS Left side represents the net rate at which x-momentum leaves the CV due to fluid motion across its boundaries. Right side represents the net pressure force and the net forces due to viscous shear stress What special approximations may be made for conditions within theis velocity and thermal boundary layers? u>>v du/dy>>du/dy(ρv/dx) dt/dy>>dt/dx How is the Reynolds # defined? What is it physical interpretations? What role is played by the critical? Re= Vx/υ ratio of inertial and viscous forces calculates the boundary layer behavior What is the Prandtl #? How does its value affect relative growth of the velocity and thermal boundary layers for laminar flow over a surface? What representative room temperature values of the Prandtl # for a liquid metal a gas water and oil Pr= υ/α ratio of the momentum and thermal diffusivities Prandtl # for different fluids is different, for energy and momentum transfer by diffusion rate is as follows; gases its comparable, liquid metal – energy exceeds momentum diffusion rate and oils are opposite of liquid metal What is the coefficient of friction? The Nusselt #? For flow over a prescribed geometry what are the independent parameters that determine local and average values of these quantities? Dimensionless surface shear stress Cf= τs/ρv62/2 The ratio of convection to pure conduction heat transfer rates Nu= hL/k Under what conditions may velocity and thermal boundary layers be termed analogous? What is the physical basis of analogous behavior? If two or more processes are governed by dimensionless equations of the same form Friction and heat transfer relations for a particular geometry are interchangeable What important boundary layer parameters are linked by the Reynolds analogy? Velocity parameter- if known the analogy may be used to obtain the heat transfer parameter and vice versa What physical features distinguish a turbulent flow from a laminar flow? Laminar- fluid flow is highly ordered and you could identify streamlines along which fluid particles move Turbulent- fluid flow highly irregular and is characterized by random, 3-d motion of relatively large parcels CH7 What is an external flow? Boundary layers that develop freely without constraints imposed by adjacent surfaces How does the velocity boundary layer thickness vary with distance from the leading edge for laminar flow over a flat plat? For turbulent flow? What determines the relative velocity and thermal boundary layer thickness for laminar flow? For turbulent flow? Increases proportionally to x^1/2 Increases proportionally to x^4/5 Pr- momentum and thermal diffusivity ratio Boundary layer development is influenced by random fluctuations in fluid in the fluid and not by molecular diffusion How does the local convection heat transfer coefficient vary with distance from the leading edge for laminar flow over a flat plat? For turbulent flow? For flow in which transition to turbulence occurs of the plate Decrease proportionally to x^-1/2 from ω Decrease proportionally to x^4/5

Decrease in a range from xlam