 Momentum Heat Mass Transfer

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Momentum Heat Mass Transfer
MHMT11 Heat transfer-convection Thermal boundary layer. Forced convection (pipe, plate, sphere). Natural convection. Rohsenow W.M., Hartnett J.P., Cho Y.I.: Handbook of Heat Transfer. McGraw Hill, 3rd Edition, 1998 Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

Heat transfer - convection
MHMT11 Heat flux from a surface to fluid is evaluated using heat transfer coefficient , assuming linear relationship between heat flux and driving force, which is the temperature difference between solid surface and temperature of fluid far from surface (outside a thermal boundary layer) Heat transfer coefficient is related to thickness of thermal boundary layer y T T Tw x

Thermal boundary layer
MHMT11 Integral boundary layer theorem can be derived in the same way like for the momentum boundary layer Upper bound H 

Thermal boundary layer
MHMT11 Final form of integral boundary equation This equation can be applied as soon as the momentum boundary layer was identified (velocity profile inside the thermal boundary layer must be known). This equation is used for prediction of heat transfer at flows along plates, pipes, cylinder, spheres… However, in the following slides we shall analyze heat transfer in a pipe and along a plate in a simpler way, using the penetration theory (instead of integral theorem). The penetration theory predicts not so accurate but still qualitatively correct results.

Heat transfer – pipe MHMT11
Forced convection: Reynolds number is given, Nusselt number is to be calculated. Duchamp

Heat transfer – pipe (laminar)
MHMT11 Convective heat transfer in a pipe for the laminar flow and fully developed velocity profile (therefore it is not necessary to solve the problem of momentum boundary layer). penetration theory Velocity profile y x Tw T0 D parabolic velocity profile slope of the velocity profile behind the boundary layer is the inlet temperature T0 Leveque formula Remark: This correlation predicts local value of (x), which is stressed by lower index in Nusselt and Graetz number

Heat transfer – pipe (laminar)
MHMT11 Anyway, qualitatively the same result can be obtained using integral theorem assuming linear temperature and velocity profile in the thermal boundary layer …this results differs only by a constant (9/8) from the previous result (1/2) but this constant is anyway usually modified using experiments or more accurate assumptions.

Heat transfer – pipe (laminar)
MHMT11 Local value (x) increases to infinity with decreasing distance from the tube inlet. From practical point of view a more important is the average value of  at pipe of the length L . index ln is used because this ln is related to the mean logarithmic temperature difference used in the heat exchanger design Correlations used in practice for heat transfer prediction in laminar flows in pipes Léveque formula for Gz>50 (short pipes) Hausen formula for arbitrary long pipes Note the fact that at very long tubes the Nu (and ) is constant 3.66, and that the Hausen correlation reduces to Leveque for Gz

Heat transfer – pipe (turbulent)
MHMT11 Turbulent flow is characterised by the energy transport by turbulent eddies which is more intensive than the molecular transport in laminar flows. Heat transfer coefficient and the Nusselt number is therefore greater in turbulent flows. Basic differences between laminar and turbulent flows are: Nu is proportional to in laminar flow, and in turbulent flow. Nu doesn’t depend upon the length of pipe in turbulent flows significantly (unlike the case of laminar flows characterized by rapid decrease of Nu with the length L) Nu doesn’t depend upon the shape of cross section in the turbulent flow regime (it is possible to use the same correlations for elliptical, rectangular…cross sections using the concept of equivalent diameter – this cannot be done in laminar flows) The simplest correlation for hydraulically smooth pipe designed by Dittus Boelter is frequently used (and should be memorized) m=0.4 for heating m=0.3 for cooling

Heat transfer - plate MHMT11
Analysis of heat transfer at external flows (around a plate, cylinder, sphere…) differs from internal flows (for example heat transfer in pipe) by the fact that velocity profile at surface is not known in advance and therefore not only the thermal boundary layer but also the momentum boundary must be solved Duchamp

Heat transfer - plate MHMT11
Heat transfer in parallel flow along a plate is characterised by simultaneous development of thermal and momentum boundary layers. It will be assumed, that the thickness of momentum boundary layer H is greater than thermal boundary layer T Linear velocity profile (this is previously obtained result for thickness of the momentum boundary layer) Thermal boundary layer thickness T y x Tw T H Note the fact, that the ratio of thermal and momentum boundary layer thickness is independent of x and velocity U

Heat transfer - plate MHMT11
Mean value of heat transfer is obtained from the previous formula by integration along the length of plate L y x L Laminar flow regime Turbulent flow regime (Re> ) Compare these results with the heat transfer in pipe Pipe: Laminar flow NuRe1/3 turbulent flow NuRe0.8 Plate: Laminar flow NuRe1/2 turbulent flow NuRe0.8

Heat transfer – sphere…
MHMT11 Flow around a sphere (Whitaker) Rear side (wake) Front side Important for heat transfer from droplets… See next slide 0,71Pr ,5Re7, Cross flow around a cylinder (Sparrow 2004) Important for shell&tube and fin-tube heat exchangers 0,67Pr Re105 . Cross flow around a plate (Sparrow 2004)

Heat transfer – sphere MHMT11
Conduction outside a sphere, see 1D solution of temperature profile Tw T D r

Natural convection MHMT11
Velocity and Reynolds number is not known in advance. Flow is induced only by buoyancy. Instead of Reynolds it is necessary to use the Rayleigh number. Duchamp

Natural convection MHMT11
Vertical wall. Wall temperature Tw>Tf. Heated fluid flows upward along the plate due to buoyant forces. um x y Forces equilibrium (wall shear stress=buoyant force) Penetration depth at distance x Substituting um into force balance where Ra is Rayleigh number (x represents a characteristic dimension in the direction of gravity, e.g. height of wall)

Natural convection MHMT11
Turbulent flow regime occurs at very high Ra>109 and instead the 4th root of Ra the 3rd root is used in correlations Note the fact, that at turbulent flow regime the heat transfer coefficient is almost independent of x (size of wall)

EXAM MHMT11 Forced convection Natural convection

Forced convection Nu(Re,Pr)
What is important (at least for exam) MHMT11 What is it heat transfer coefficient and how is related to thermal boundary layer thickness Forced convection Nu(Re,Pr) Heat transfer in a pipe (laminar) Heat transfer a plate (laminar) Heat transfer turbulent

Free convection Nu(Ra)
What is important (at least for exam) MHMT11 Free convection Nu(Ra) Laminar flow (Re<1010) Turbulent flow

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