Heat Flashcards
Heat
Thermal energy in motion
Heat transfer - no medium required
Radiation
Heat transfer - If the energy is transferred through a solid material by molecular vibration
Conduction
Heat transfer - Energy transferred from one point to another by a moving fluid
Convection
What type of convection transfers heat by relying on density changes to cause fluid motion
Natural Convection
What type of convection requires a pump, fan, or relative motion to move the fluid
Forced Convection
The rate of heat transfer will initilly vary with time. The initial period is known as
Transient Period
De que es esta formula: Q=-kA(T2-T1)/L
Conduction through a flat slab
T - surface temperatures
De que es esta formula: R = L/kA
Thermal Resistance
De que es esta formula:
Q= T1-T2/Ra = T2-T3 / Rb
Conduction Heat transfer through Composite slabs
Thermal Resistances are in series - the total thermal resistance of a composite slab is the sum of the individual thermal resistance.
De que es esta formula:
Q= 2pikL(T1-T2) / ln(r2/r1)
Conduction through composite cylinders
Logarithmic mean area should be used with heat transfer through thick pipes and cylindrical tank walls.
Am= A2-A1/ln (A2/A1) = 2piL(rw=r1)/ln(r2/r1)
Thermal Resistance in natural convection (cual es el coefficient)
Based on film coefficient (h)
Film coefficient (h) formula
h=Q/ A(Tw - Tinf)
Tw = surface temp
Tinf - Local Temp (environment)
Depending on the heat transfre path, the total thermal resistance may include both conductive and convective compoents.
If they are in series they ____
If they are parallel, they ___
If they are in series the thermal resistance add
If they are parallels, their reciprocals add
The overall coefficient of heat transfer, U, is the reciprocal of teh total thermal resistance
1/U = Sum Ri = Sum Li/ki + Sum 1/hj
Newton Law of Convection calculates the forced convective heat transfer. Formula
Q= hA(Tw-Tinf)
Raditiation directed to a body can be
absorbed, reflected or transmitted
Opaque solids & some liquids tiene transmissivity of
T (transmissivity) - 0
Gas reflect very little radiant energy tienen rho
rho=0
Black bodies
absorbs all the radiant energy
Emissivity of a gray body
Qgray/Qblack = acutal radiation emitted to that emitted by a black body
Radiant Heat transfer formula
Q = esigmaAT^4
Q1-2 = A1F12sigma(T1^4-T2^4)
sigma=5.670 x 10^-8
F12 = shape factor
If body 1 is small and completely enclosed by body 2, then F12=E1
All forms of radiation travel through a vaccum (T o F)
True
