Week 4 - CHT Analysis on Exhaust port
USED OF CHT ANALYSIS
Conjugate heat transfer is the phenomena of simulating heat transfer in both the fluid flow to solid heat transfer. i simunltenously solves all revelent flow process wquation.
efficiently combining heat transfer in fluid and solids in the key to designing effective coolers , heaters, heat exchangeers.
heat transfer in fluid and solids can be also combined to minimize heat losses in various devices. as most gases have small thermal conductivies they can be used as thermal insulators if they are not in motion.
in heat engine parts such as in exhaust port cyclinder or wherever heat transfer occurs this calculation helps in desighn and selection of materials properties based on output of the simulation.
effectiveness cooling strategies are critical for various components for emobility systems. electric motors have high power density and thus generate a significant amount of heat . if motor is not properly cooled high temperature in the motor can cause the demagnetization of permemnant magnet and breakdown of winding insulation.
resolving the spacially varides useful temperature data.
SIMULATION PROCEDURE:
1. Load the geometry into spaceclaim
we load geometry then we use shareprep option from workbench when we use this mesh boundary of the bodys in contact( solid and fluid) they are perfectly interfacing their edges with each other so it creates the conformal mesh there will not be any interfence between meshing of solid and fluid meaning zero distance between two meshes.
when we hit share this red coloured boundary shown where the faces of meshing boundary of two bodies are interfacing with each other.
and surface boundary of two faces become pink means share option is enabled and the boundary is shared.
2. in the mesh we first create baseline meshing then as we observe that not finer mesh we required sizing and inflation
here the yplus is very important to understand . so we want to catch the physics near the wall . we want to model total turbulence layer hence let us assume first node to be viscous sublayer.as yplus of 1 to 5 gives the node in viscous sub layer we assume Yplus 5 in this case and be below calculation let us consider the first layer thickness.
so here the first layer thickness is 0.26 mm.
3. set up
here we provide the information or inputs for the simulation. so when we open setup we see vlue arrows for inlets and red for outlets.
general setting:
we are going to set viscous model tp K-w aswe want to capture near wall effects.
boundary condition are as below
inlet as velocity with v=5 m/s and temp as 700 k under thermal tab.
here for wall heat transfer mode is going to be convection and we are considering coefficient 20 W/m2K. generally finding h is not so easy task, it depend supon the experimental conditions. or we have to take another CFD simulation.
next is material we have used air for fluid material and aluminium for solid which were default values.
after this we care createing contour of velocity and temperature.
results:
a. temperature contour:
residual:
velocity contour:
streame line:
temperature plane:
velocity plane:
we can see that there is much more velocity accounted at red region in contour. hence there will be more reynolds number more heat transfer coefficient also.
CFD post
h value at inlet
h value at outlet:
case 2
now we refine the mesh by redusing the y+ value of wall from 5 to 1.
in this case set up and geometry remains the same meshing and solution changes.
scale residual:
temperature countour:
validation of CFD simulation
let us consider one case out of the above two validation purpose.
here we have consider y plus of 1 and below are the values for analytical calculation of h at inlet and outlet.
Heat transfer coefficient at inlet and outlet
Nu= hL/k
h=k.Nu/L
where k is a thermal conductivity of fluid =0.025 w/m-k
L, is hydraulic diameter
Nu is nusselt number
Nu=0.023*Re^0.8*pr^0.3
Re is reynolds number and Pr is prandtl no.
Re=Ro*v*D/Mu
=1.225*5*0.2/1.789e-5
=6.8e+4
Pr=Mu*Cp/k=1.789e-5*1075/0.025=0.7949
Nu=157.699
h=19.71 W/m^2-k
so, at the inlet velocity is much less hence cause less heat transfer.
Heat transfer coefficient at outlet:
Nu= hL/k
h=k.Nu/L
where k is a thermal conductivity of fluid =0.025 w/m-k
L, is hydraulic diameter
Nu is nusselt number
Nu=0.023*Re^0.8*pr^0.3
Re is reynolds number and Pr is prandtl no.
A2*v2=A1*v1
A2=4*A1
so,V2=4*v1=20 m/s
Re=Ro*v2*D/Mu
=1.225*20*0.2/1.789e-5
=2.72e+5
Pr=Mu*Cp/k=1.789e-5*1075/0.025=0.7949
Nu=478.05
h=59.756 W/m^2-k
so at the outlet velocity is high which means a high reynolds number which leads to a high heat transfer rate, hence high heat transfer coefficient
above analytical calculation we did is for simple pipe flow . but we did simulation of exhause pipe which is much complicated geometry and many bends are there in between so we can never expect the accurate comparision but we can definetly know the range of results.
comparison table for h value from simulation:
| simulation no | Yplus | first layer thickness(mm) | surface heat transfer coefficient inlet by simulation | surface heat transfer coefficient outlet by simulation |
| 1 | 5 | 0.26 | 53.822 | 140.709 |
| 2 | 1 | 0.052 | 34.536 | 76.701 |
CONCLUSION:
* For turbulance model in program we have to take care of y+ value and boundary conditions. meshing also plays importance role Coarse mesh will not give accurate results. so we have to refine geometry to catch importance phenomena that are happening near wall or some special areas
*There are few factor which we have to consider which are significant are turbulent boundary layer Y+ value . so when we are using RANS model, in that k-w SST model we need to stay in viscous sub layer of turbulent boundary layer where velocities are assumed to be laminar and viscous stress dominates the wall shear meaning Y+ <5.
*Motivation to use k-w model instead of k-e is to address the shortcoming of k-e model in dealing with near wall treatment . so k-e works well for those flows where we want to capture only those regions where adverse pressure gradient are not present and those are core turbulent flows. so omega based model use blending function F1 which is best for core turbulent flow and near wall also.
* in fluent reference temperature is importance in determining heat transfer coefficient.