Experimental and theoretical investigations of falling film evaporation

Abstract

In this study, a mathematical model was developed for falling film evaporation in vacuum using heat transfer relations. An experimental device was designed. experimental set-up which was used was equipped with a triangular weir distribution device and it had the ability to record data up to 3 m. Experiments were performed in a single-effect process with sucrose-water solution varying from 3 to 20% concentration rate of sucrose and we used a vertical tube evaporator with the dimensions of laboratory scale. The model that was developed considers convection, shear stress, viscosity and conjugate heat transfer while most of the previous works ignored these factors. The main factors influencing the heat transfer mechanism performance of the unit were investigated and analyzed. We concluded that the experimental studies are verified by the developed model. Furthermore, it was also concluded that, the heat transfer is affected by the mass flow rate, sucrose concentration rate in solution, film thickness and pressure. List of symbolsA Area (m 2 ) c Concentration (kg/kg) d Thickness, diameter (m) f Interfacial friction factor g Gravity (m/s 2 ) h Heat transfer coefficient (W/m 2 K) h High (m) i Enthalpy (kJ/kg) k Heat transfer coefficient (W/m K) Ka Kapitza number l Characteristic length (m) _ m Mass flow rate (kg/s) Nu Nusselt number Pe Peclet number Pr Prandtl number q Heat flux (W/m 2 ) Q Heat (W) Re Reynolds number t Time (s) T Temperature (°C) U Overall heat transfer coefficient (W/m 2 K) V Velocity, liquid film velocity (m/s) _ V Volumetric rate (m 3 /s) W Length (m)Greek letters l Dynamic viscosity (Pa s) m Kinematic viscosity (m 2 /s) s Shear stress (N/m 2 ) q Density (kg/m 3 ) d Film thickness (m) a Heat transfer coefficient (W/m 2 K) k Heat transfer coefficient (W/m K)

Pilot-scale investigation of effectiveness of evaporation of skim milk compared to water

“…where U is the overall heat transfer coefficient (kW•m −2 Á°C -1 ), Q the heating power (kW), A the heating surface (m 2 ), Δθ the mean temperature difference between steam and product concentrate (°C), and Ф the heat flux (kW•m −2 ). The overall heat transfer coefficient is an overall resistance resulting from the combination of the resistance to heat transfer on the steam heating side, the tube wall thickness, a fouling layer on the product side, and the interface with the boiling product (Pehlivan and Özdemir 2012;Adib et al 2009;Adib and Vasseur 2008;Prost et al 2006;Bouman et al 1993).…”

“…It should be remembered that the evaporation rate is related to the operation conditions (e.g., distribution system, heat flux, etc.) (Morison et al 2006) and product characteristics, such as viscosity and surface tension (Pehlivan and Özdemir 2012;Weise and Scholl 2009;Morison et. al.…”

“…As boiling and concentration take place inside the falling-film evaporator, the study of the mechanisms is complex because both occur simultaneously (Li et al 2011;Pehlivan and Özdemir 2012). Some studies of the evaporation process reported in the literature were carried out on industrial evaporators.…”

Optimization of process variables on physicochemical properties of milk during an innovative refractance window concentration

“…Overall heat transfer coefficient was calculated using Equation (3) (Pehlivan & Özdemir, 2012). where, U is the overall heat transfer coefficient (W/m 2 °C), A is the area (m 2 ), ΔT m is the logarithmic mean temperature difference (LMTD).…”

Influence of Mechanical and Thermal Boundary Conditions on Stabilizing/Destabilizing Mechanisms in Evaporating Liquid Films

“…Besides coatings, liquid films find applications in the food processing industry [5,6], cooling of server towers that was a $36 Billion industry in 2007 [7][8][9] and for improved oil recovery from petroleum reservoirs [10,11].…”