Abstract
The aim of this paper is to describe the variation laws of the boiling heat transfer coefficient ( h) versus the main process parameters, using a pilot scale falling film evaporator as found in many food industries. Sugar solutions at different concentrations are used as a model of Newtonian liquid food. The studied parameters affecting boiling heat transfer coefficient ( h) in the falling film evaporator are: the dry matter concentration X DM (or Brix for sugar solution), the evaporating temperature ( θ L) or pressure ( P) taking into account the boiling point elevation (BPE), the heat flux ϕ or the temperature difference between the heated surface and boiling liquid temperature (Δ θ) and the specific mass flow rate per unit of perimeter length ( Γ). The nature of heated surface is kept constant (stainless steel) and the effect of the emitted vapor velocity is not taken into account in our study. The variations of h with ϕ or Δ θ, are given for pure water and sugar solutions at different concentrations (10%, 30%, 50% and 70%), and interpreted in relation with the two boiling regimes (non-nucleate and nucleate). The transition between non-nucleate regime and nucleate regime has also been visually observed. The critical specific mass flow ( Γ cri) for water and sugar solution at dry matter concentration 50% has been studied. The variations of h in function of X and θ L at Γ = 0.56 (kg m −1 s −1) at three different heat fluxes (2, 20 and 80 kW m −2) have been presented. Two models to correlate h in function of viscosity at Γ = 0.56 (kg m −1 s −1) at two different heat fluxes (20 and 80 kW m −2) have been established, and have been compared with a bibliographic model presented by Van der Poel et al. [P.W. Van der Poel, H. Schiwec, T. Schwartz, Sugar Technology Beet and Cane Sugar Manufacture, Verlag Dr. Albert Bartens KG, Berlin, 1998]. In this paper, experimental results and experimental models, for prediction h in sugar solutions in function of X DM, θ L (°C), ϕ (kW m −2) and Γ (kg m −1 s −1) are presented.
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