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Experimental investigation of combined forced and free convection in horizontal and inclined tubes

Sommario

L'indagine é volta alla determinazione sperimentale dei coefficienti di scambio termico in regime di convezione mista per acqua in moto laminare entro un condotto a sezione circolare. Allo scopo di consentire il confronto dei risultati con le previsioni teoriche, l'esperienza è stata condotta cercando di approssimare le condizioni al contorno di flusso termico uniforme lungo l'asse del tubo e di temperatura uniforme in senso circonferenziale. La dipendenza dei coefficienti di scambio termico dai numeri di Reynolds e di Rayleigh è stata analizzata sia per condotto orizzontale che inclinato. La sperimentazione ha coperto il campo di valori compreso tra 200 e 2300 del numero di Reynolds all'ingresso, e tra 6000 e 70000 del numero di Rayleigh. L'influenza dell'inclinazione sullo scambio termico è stata esaminata, per moto laminare ascendente, tra 0 e 60° sull'orizzontale.

Summary

This investigation considers the effects of free convection on the laminar flow of water through a circular duct having essentially constant wall heat transfer rate per unit length of the duct and circumferentially uniform wall temperature. The effect of the Reynolds and Rayleigh number variations on heat transfer results has been analysed for both horizontal and inclined pipe. The experiment has covered the range of the inlet Reynolds number from 200 to 2300, and of the Rayleigh number from 6,000 to 70,000. The effect of pipe inclination has been investigated for slope angle values up to 60°, with laminar ascending flow.

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Abbreviations

c p :

specific heat

d :

inside tube diameter

Gr :

Grashof number,gβd 3(T w T f,b )/v 2

Gr q :

modified Grashof number,GrNu=gβd 2 q w /(v 2 k f )

g :

gravitational acceleration

h x :

local heat transfer coefficient

k f ,k w :

fluid and tube wall thermal conductivity

l :

length of heat transfer section

m :

mass flow rate

Nu x :

local Nusselt number,h x d/k f

Nu m :

mean Nusselt number, defined by eqn. (5)

Pr :

Prandtl number,c p μ/k

P w :

tube wall parameter,hd 2/(k w t

P * w :

modified tube wall parameter,P w /Nu=k f d/(k w t))

q Q :

heat input per unit length

q w :

heat flux

Ra :

Rayleigh number,GrPr

Ra q :

modified Rayleigh number,Gr q Pr

Re :

Reynolds number,\(\bar ud/v\)

T f :

fluid temperature

T w :

local wall temperature

t :

tube wall thickness

\(\bar u\) :

average velocity

x :

downstream distance

x * :

reduced downstream distance,x/(dRePr)

α:

inclination angle over the horizontal

β:

isobaric thermal expansion coefficient

θ:

angle measured clowkwise from top of tube section

μ:

dynamic viscosity

ν:

kinematic viscosity

ϱ:

density

as :

asymptotic

b :

bulk

i :

inlet

o :

outlet

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Research supported by C.N.R., grant no. 81.002.77.07.

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Barozzi, G.S., Zanchini, E. & Mariotti, M. Experimental investigation of combined forced and free convection in horizontal and inclined tubes. Meccanica 20, 18–27 (1985). https://doi.org/10.1007/BF02337057

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  • DOI: https://doi.org/10.1007/BF02337057

Keywords

  • Heat Transfer
  • Reynolds Number
  • Laminar Flow
  • Allo
  • Rayleigh Number