Physics

Physics                                                    


 

   Preliminary studies on a particular case

This first study involves to work on evaporation in enclosure where a  pure macro-layer is lied. This enclosure is considered adiabatic except the bottom wall where its temperature is fixed. For the first part, the study is in 2D but the edge effect along the vertical wall is neglected.

The second study involves to work on a particular case with different boundary layer : thermal and mass boundary layer.

 

Problem modeling

Problem modeling                             


 

       1- Lists of symbols & subscripts

               Symbols:

Here are the list of all symbols and subscripts used in the study.

  • $h$    liquid thickness
  • $L$    container height dimension
  • $l_c$    capillary length
  • $a$    depth
  • $b$    length
  • $S_{flux}=a b$    flux surface ($m^2$)
  • $\rho$    density of the gas mixture ($kg/m^3$)
  • $P$    pressure ($Pa$)
  • $u$    the projection of velocity vector along the x-axis ($m/s$)
  • $\mu$    the dynamic viscosity of the gas mixture  ($Pa.s$)
  • $\nu$    the cinematic viscosity of the gas mixture ($m^2/s$)
  • $v$    the projection of velocity vector along the y-axis ($m/s$)
  • $g$    standard gravity ($m/s^2$)
  • $C_p$    specific heat ($J.kg^{-1}.K^{-1}$)
  • $T$    temperature of the mixture ($K$)
  • $\lambda$    the thermal conductivity ($W/m.K$)
  • $D$    diffusion coefficient of the vapour in the air ($m^2/s$)
  • $\rho_S$    mass concentration of vapour at the interface ($kg/m^3$)
  • $\omega$    mass fraction of vapour in the air with $\omega= \frac{\rho_v}{\rho_v+\rho_a}$
  • $\dot{m}$    mass flow rate ($kg/s$)
  • $h_{lg}$    latent heat ($kJ/kg$)
  • $h_c$    thermal exchange coefficient
  • $h_m$    mass exchange coefficient
  • $D_h$    hydraulic dimension

‚Äč

                Subscript :

  • $v$    for the vapour
  • $S$    for the vapour at the interface
  • $in$    for the input condition
  • $w$    for the wall condition
  • $l$   for the liquid

 

        2- Assumptions related to the vapour- state :

We consider a layer of liquid in a ventilated box. The following figure represents the problem:

 

           

                                     Figure: Problem modeling & Simplifications

 

During this study, we simplify the problem by considering the following assumptions:

  • Bidirectional incompressible turbulent flow.
  • Liquid film thin in relation to the reservoir but sufficient thick for neglect the interfacial resistance.
  • Air charged in vapour is in thermodynamic balance : the phase change happens in saturation conditions.
  • Viscous dissipation and pressure work is neglected.
  • Soret effect ( temperature gradient dependence for the mass flow) and Dufour effect (mass gradient dependence for the heat flux) neglected.
  • Radiation phenomena neglected.
  • Boussinesq approximation considered.

In further, the conservation equations are presented below.

Equations

Equations                                                


 

       1- Non-dimensional equations

For a greatest lisibility of the mechanisms, the non dimensional equation can be written as :

$$X=\frac{x}{L} , Y=\frac{y}{L} , U=\frac{u}{U_{in}} , V=\frac{v}{V_{in}}, D_{hydraulic}=L$$ ,

$$ \theta=\frac{T-T_{in}}{T_s-T_{in}} ,  C=\frac{\omega-\omega_{in}}{\omega_s-\omega_{in}}$$

The non dimensional equations become :

$$U\frac{\partial U}{\partial X}+V\frac{\partial U}{\partial Y}=\frac{1}{Re}\left(\frac{\partial^{2}U}{\partial X^{2}}+\frac{\partial^{2}U}{\partial Y^{2}}\right)    (1*)$$

$$U\frac{\partial V}{\partial X}+V\frac{\partial V}{\partial Y}=-\frac{\partial P_{m}}{\partial Y}+\frac{1}{Re}\left(\frac{\partial^{2}V}{\partial X^{2}}\right)      (2*)$$

$$U\frac{\partial \theta}{\partial X}+V\frac{\partial \theta}{\partial Y}=\frac{1}{RePr}\left(\frac{\partial^{2}\theta}{\partial X^{2}}+\frac{\partial^{2}\theta}{\partial Y^{2}}\right)    (3*)$$

$$U\frac{\partial C}{\partial X}+V\frac{\partial C}{\partial Y}=\frac{1}{ReSc}\left(\frac{\partial^{2}C}{\partial X^{2}}+\frac{\partial^{2}C}{\partial Y^{2}}\right)      (4*)$$

 

        2- Non-dimensional numbers

Some interesting numbers appear in this non-dimensional equation.

  • Reynolds number

The Reynolds number is characteristic of pure flow conditions, inertial effect compared with momentum diffusion:

$$Re_{in}=\frac{\rho_{in}LU_{in}}{\mu}$$

  • Prandlt number

Another one is the Prandlt number. It appears in thermal equation and it compares the momentum diffusion with the thermal diffusion

$$Pr=\frac{\frac{\mu}{\rho_{in}}}{\frac{\lambda}{\rho C_p}}$$

  • Schimdt number

The Schimdt number, quiet close with the Prandt number but it appear in mass equation. It compares the movement quantity diffusion with the molecular diffusivity.

$$Sc=\frac{\frac{\mu}{\rho_{in}}}{D}$$

In addition the expression of the thermal flows through the liquid film has to be written clearly because the study of the heat transfers can explain the energy transfer between the liquid phase and the vapour phase.

Firstly, the sensible heat flux can be modeled as:

$$ q''_s=-\lambda_{liq} \left(\frac{\partial T}{\partial y}\right)_{y=h}          (5) $$

That is the energy transfer through the thickness of the liquid  resulting from temperature gradient. We can suppose in this case the convection is neglected.

 

Then the local energy balance on the interface is :

$$q''_{evap}=q''_{fluid}+q''_ {gas}$$

that is to say :

$$h_{lg}h_m(\rho_{in}-\rho_{S})=h_c(T_{in}-T_S)+\lambda_l \left(\frac{\partial T}{\partial y}\right)_{y=h}$$

  • Nusselt number & Sherwood number

After all that, the Nusselt number and the Sherwood number are interesting to be studied. These numbers compare the convective effect and the diffusion effect.

The local Nusselt number compares the thermal convection flux with the thermal diffusion flux and it is defined as :

$$ Nu=\left(\frac{\partial \theta}{\partial Y}\right)_{Y=h/L}$$

Then we can deduce the global Nusselt Number as :

$$\bar{Nu}=\frac{\bar{h_c}*D_h}{k}=\frac{q_t*D_h}{\lambda(T_{in}-T_{S})}$$

 

To finish this first part, the Sherwood number compares mass convective flux with molecular diffusion.

$Sh=\frac{h_m*D_h}{D}$ with $h_m=\frac{\dot{m}}{\rho(\omega_{in}-\omega_{S})}$

The total flux of vapour mass is $\dot{m}=\rho_l v_l=\rho_S v_S=\omega_S\rho v_S-\rho D(\frac{\partial \omega}{\partial y})_{y=h}$.

 

     N.B:

This work was made according to the thesis "Transfert couplé de chaleur et de masse par convection mixte avec changement de phase dans un canal" fulfilled by Othmane OULAID.