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FST1103 what to look out for (heat transfer(7/8) (convective heat transfer…

- - - - ratio of the actual mass of a particle to its actual volume
    - - mass of particles occupied by a unit volume of bed
    - - density relative to water
        
        specific gravity x density of water = density of the fluid
  - - - mass of water over moist sample
    - - mass of water over mass of dry solids
  - - - value of a property depends on the extent or the size of a system
      - parameters are additive
      - mass, length, volume, and energy
    - - Property does not depend on the size of a system
      - temperature, pressure and density
- - - - resistance to flow of steam to the various locations
      - loss of thermal energy or heat content during transport
    - - temperature of steam
      - quantity of steam
- - - - i. a term expressing the energy of a fluid
      - ii. expressed in meters of liquid
    - - i. all the energy, terms in head, at the suction side of the pump
    - - i. all the energy, terms in head, at the discharge side of the pump
- - - - mechanical means
    - - density difference caused by temperature gradients
      - N_Nu=(hd_c)/k=a〖(N_Ra)〗^m
        
        a and m are constant obtained from table based on the objects geometry
        
        N_Ra=N_Gr×N_Pr
      - Grashof number, NGr
        
        whether a flow over an object is laminar or turbulent
        
        NGr > 10^9 for fluid flow over vertical plates signifies a turbulent flow
        
        film temperature, T_f=(Ts+T∞)/2
        
        table should be used - different shapes/ position result in different equation or numbers to be used
        
        N_Gr=(〖d_c〗^3 ρ^2 gβ∆T)/μ^2
        
        characteristic of the liquid/gas should be obtained after determining the film temperature from the given table
    - - depends on
        
        properties of fluid
        
        specific heat
        
        viscosity
        
        density
        
        thermal conductivity
        
        velocity of fluid
        
        geometry
        
        roughness of solid object surface in contact with the fluid
      - finding h
        
        Identify flow geometry
        
        Identify the fluid and determine its properties
        
        Calculate the NRe or NGr (free convection)
        
        calculated nusselt number using corresponding equation with reference to type of fluid and geometry
        
        heat transfer coefficient can be found
        
        calculated ave temperature
    - - enhancement in the rate of heat transfer caused by convection over the conduction mode
      - Nusselt number = 1
        
        no improvement due to convection
      - Nusselt number = h x dc / k
      - flow in pipes
        
        laminar flow (Reynolds number < 2100)
        
        NNu=1.86(NRe×NPr×dc/L)^0.33×〖(μb/μw )〗^0.14
        
        2100< Reynolds number < 10000
        
        NNu=((f/8)(NRe-1000) NPr) / (1+12.7(f/8)^0.5 (〖NPr〗^(2⁄3)-1))
        
        smooth pipes
        
        f=1/〖(0.790 x lnNRe^(-1.64)〗^2
        
        Reynolds number > 10000
        
        N_Nu=0.023〖N_Re〗^0.8×〖N_Pr〗^0.33×〖(μ_b/μ_w )〗^0.14
        
        μb = viscosity at average bulk fluid temperature
        μw = viscosity at inside pipe surface temperature
      - flow past immersed objects
        
        heat transfer depends on the geometrical shape of the object, relative position of the object, proximity of other object, flow rate, and fluid properties
        
        sphere
        
        N_Nu=2+0.6〖N_Re〗^0.5×〖N_Pr〗^(1⁄3)
        
        1<N_Re<70000
        0.6<N_Pr<400
        dc, is the outside diameter of the sphere
        
        fluid properties are evaluated at the film temperature Tf
        
        T_f=(T_s+T_∞)/2
      - NNu = C x (NRe)^m x (NPr)^n
    - - ratio between the molecular diffusivity of momentum to the molecular diffusivity of heat
      - thickness of the hydrodynamic boundary layer compared with the thermal boundary layer.
      - NPr = (μ x cp) / k
      - NPr = 1
        
        thickness of -->
        hydrodynamic boundary layers = thermal boundary layers
      - NPr << 1
        
        molecular diffusivity of heat will be much larger than that of momentum
        
        heat will dissipate much faster
      - NPr is about 0.7 for gases, and around 10 for water
    - - dc=(2W×H)/(W+H)
  - - - qx = - k x A x (dT/dx)
        
        rectangular slab
        
        qx = -k x A x ((T2 - T1)) / ((x2 - x1))
        
        T - T1 = [qx / k x A] (x - x1 )
        
        qx = (T1 - T2) / Rt
        
        Rt = ((x2 - x1)) / kA
        
        conduction through multiple layers
        
        temperature difference divided by the sum of the various thermal resistance
        
        walled cylinder
        
        thermal resistance
        
        [ ln⁡(ro /ri) / 2π x L x k ]
        
        qr = (Ti - To) / Rt
        
        T2= T1 - q x [ln⁡(r2/r1 ) /2πLk]
- - - - bridge across plate contact points
    - - decrease heat transfer rate
      - deposition of solid food
  - - - T(H,inlet) - T(C,inlet)=∆T1
      - T(H,exit) - T(C,exit)= ∆T2
    - - T(H,exit) - T(C,inlet) = ∆T1
      - T(H,inlet) - T(C,exit) = ∆T2
  - - - [known ] temperatures of the fluid streams at the inlet and exit
      - [to be found] size of the heat exchanger - heat transfer area, length, and diameter of the pipe
    - - [known] size of the heat exchanger, and inlet temperatures
      - [unknown] exit temperatures of the two streams
      - approach
        
        heat capacity rate ratio
        
        C*= Cmin / Cmax
        
        heat capacity rate
        
        hot stream
        
        CH = mH x CpH
        
        cold stream
        
        CC = mC x CpC
        
        product of the mass flow rate and specific heat capacity of a liquid stream
        
        the smaller of the two quantities is called Cmin and the larger Cmax
        
        heat exchanger effectiveness
        
        εE = qactual / qmax
        
        ratio of the actual rate of heat transfer accomplished and the maximum attainable rate of heat transfer for a given heat exchanger
        
        qactual = CH x (T(H,inlet) - T(H,exit) ) = CC x (T(C,exit) - T(C,inlet) )
        
        qmax = Cmin x (T(H,inlet) - T(C,inlet) )
        
        number of transfer units (NTU)
        
        (NTU) provides a measure of the heat transfer surface area for a given overall heat transfer coefficient and minimum heat capacity rate
        
        NTU = (UA)/ Cmin
- - - - usually concerned with the time for temperature change or temperature at the geometric centre of the object
  - - - k is the thermal conductivity of the solid medium dipped in a fluid
    - - k is the thermal conductivity of the fluid medium
  - - - negligible surface resistance to heat transfer (convection resistance from fluid)
      - reference line k/(h.dc) =0 for reading off relation using graph
    - - negligible internal resistance to heat transfer (conduction resistance of object)
      - interior of object near uniform temperature
        
        most solid metal objects but not with solid foods
        
        thermal conductivity of a solid food is relatively low
        
        high thermal conductivity object placed in a medium with poor conductor of heat
        
        no temperature gradeint
    - - indicates finite internal and external resistance to heat transfer
      - Heisler charts are to be used
  - - - calculate (1/biot number) and fourier number --> then find the temperature fraction
    - - calculate (1/biot number) and temperature fraction--> then find the fourier numberfraction
    - - finite objects to find the temperature for the object need to find temperature fraction for the infinite dimensions
      - cylinder
        
        infinite cylinder x infinite slab
      - slab
        
        infinite width x infinite depth x infinite height
- - - - using latent heat of refrigerant passing fridge to vapourize liquid to gas
      - Pressure is greatly reduced  causing the liquid to have lower boiling point  surrounding heat/ heat from liquid sufficient to cause the liquid to vaporize  vaporised liquid cools
    - - transfer heat from the refrigerant to another medium
      - vapour condenses
      - cooled at constant pressure
    - - [enter] refrigerant vapour state at low pressure+ temperature
      - pressure used to increase temperature
    - - a metering device that controls the flow of liquid refrigerant to an evaporator