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Estimation et lois de variation du coefficient de transfert de chaleur surface/ liquide en ébullition pour un liquide alimentaire dans un évaporateur à flot tombant

Abstract : The heat transfer coefficient value is necessary to calculate the heat exchange surface when designing an evaporator, as currently used to concentrate liquids in food industry. The boiling heat transfer coefficient on the liquid side (h) is the most uncertain and: it depends on the liquid thermo-physical properties (ηL, σL, λL, ρL , CpL,...) as well as on the process conditions (type of evaporator, φ or Δθ, Γ (δ), P, surface roughness, fouling, etc). Also, h depends on the boiling regime (non- nucleate or nucleate) and on the flow regime (laminar or turbulent) according to the film Reynolds number in falling film evaporators. The objective of our work is to define an economical and robust method to estimate h in a falling film evaporator, which is common in food industry for concentrating fruit juice, milk and sugar solutions. The first section of our study was a bibliographic analysis which revealed the important dispersion among the h values calculated from the formulas cited in literature. The second section was to design and construct a laboratory scale falling film evaporator (pilot) used to estimate h at stationary parameters conditions. The third section was to describe the results and variation laws of h versus the liquid dry matter concentration XMS, the boiling temperature θL, the heat flux φ or temperature gap Δθ and mass flow rate per unit of perimeter length Γ (with describing the critical mass flow for some solutions) noted that the nature of heating surface is kept constant during our work. We described the effect of each variable separately on h, where the other variables being kept constant. Also we studied the transition from non nucleate regime to nucleate regime, which varied with the nature of liquid and the liquid concentration. Finally, we presented the experimental models for h = f (XMS, θL, φ, Γ) for a Newtonian liquid (sugar solution) and non-Newtonian solution (CMC) that may be used for industrial evaporator design after their validation. We have also proposed a method for the simplification of the experimental design.
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https://pastel.archives-ouvertes.fr/pastel-00004544
Contributor : Ecole Agroparistech <>
Submitted on : Tuesday, January 13, 2009 - 8:00:00 AM
Last modification on : Saturday, June 6, 2020 - 5:37:24 PM
Long-term archiving on: : Friday, September 10, 2010 - 1:04:23 PM

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  • HAL Id : pastel-00004544, version 1
  • PRODINRA : 246667

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Tarif Ali-Adib. Estimation et lois de variation du coefficient de transfert de chaleur surface/ liquide en ébullition pour un liquide alimentaire dans un évaporateur à flot tombant. Sciences de l'ingénieur [physics]. AgroParisTech, 2008. Français. ⟨NNT : 2008AGPT0007⟩. ⟨pastel-00004544⟩

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