Lecture 1 & 2- Principles of convection (KARIMI) Flashcards
What is convection?
If a moving fluid is in contact with a solid or fluid at a different temperature, heat is convected by the moving fluid.
Does conduction still exist during convection?
Yes but usually in moving fluids convection is much stronger
What is newtons law of cooling?
q=hAdeltaT
q = rate of heat transfer [W]
h= convection coefficient [W/m^2.K]
A= area over which heat is transferred [m^2]
deltaT = temp difference between the solid surface and the fluid
What is a thermo-physical property?
as soon as the material type and its temp are known, the property can be found in tables.
Is the convection coefficient a thermo-physical property of the fluid?
No, it also depends on the nature of the flow
What happens when a uniform flow with constant velocity and temp flows over a flat wall (fluid is viscous)?
The first row of molecules sticks to the solid surface (viscosity) u_(y=0)=0. Friction between the first and second rows: retardation (loss of momentum). Friction between the third and second rows: less retardation. Other layers get similarly retarded. At boundary layer thickness u/u(original)=0.99
Are viscosity effects very significant inside the boundary layer?
YES, there is a large velocity gradient within the boundary layer
Equation for viscous stree?
tau = mu.du/dy
Equation for skin friction coefficient?
Cf = tau/0.5rhou(original)^2
Are viscosity effects significant outside the boundary layer?
No viscous effect, no velocity gradient, no shear stress.
How does boundary layer thickness change along the surface?
As the molecules get retarded more fluid penetrates into the BL and so boundary layer thickness increases along the surface
Does the bernoulli-eqn hold inside and outside the boundary layer?
only holds outside the BL
What happens to the temp of the stationary layer of molecules?
Take the wall temp
What happens between the retarded fluid layers?
There is conduction between the retarded fluid layers. Therefore there are strong temp gradients inside the thermal boundary layer
What is the temp at the edge of the thermal boundary layer?
(Ts-T)/(Ts-T(original)=0.99
does the thermal boundary layer grow or shrink along the wall?
Grows
Describe laminar boundary layer?
Flow is highly ordered and along well organised streamlines.
What is the transition zone?
Flow starts to become irregular and shows complex behaviour of a mix of laminar and turbulent flow.
Describe turbulent flow BL?
Disorderness, chaotic motion, high level or mixing and velocity fluctuations but the flow remains almost laminar in the vicinity of the wall (viscous or laminar sub-layer)
What happens to velocity gradient and shear stress/viscous force as BL thickness increases?
Velocity gradient decreases and so does the surface shear stress/ viscous force.
In the turbulent boundary layer is there a higher or lower convection coefficient and why?
Higher because velocity fluctuations in the turbulent boundary layer greatly enhance the transfer of heat and momentum.
Can you draw the diagrams comparing laminar and turbulent boundary layers?
YES OR NO
Equation for Reynolds number?
Re_x=rho.u(original).x/mu
Equation for critical Reynolds number?
Re_c=rho.u(original).x_c/mu
where x_c is the critical length for which transition to turbulence begins
What is Re_c?
The value of Re_x for which transition begins, Re_x flow is laminar.
Combining Newton’s law of cooling and conduction heat flux in the stationary layer gives?
h = -k_f.(dT/dy)/(Ts-T(original))
What is the non-dimensionalised coefficient of convection?
Nusselt Number : Nu=h.L/K_f h = coefficient of convection L = characteristic length defined according to the problem configuration K_f = fluid thermal conductivity
In forced convection, what is nusselt number a function of?
Nu = Nu(Re,Pr)
Re is defined as what in relation to Nu?
Nu = U.L/v <–(nu)
What is Prandtl number defined as?
Pr = v <–(alpha)
What is alpha?
Thermal diffusivity, alpha = k_f/rho.Cp
Types of properties are Pr and Re?
Pr - thermo-physical property of the fluid, Re - characteristic of the flow
