CONJUGATE HEAT TRANSFER ANALYSIS ON AN EXHAUST MANIFOLD USING ANSYS FLUENT
1. Conjugate heat transfer:
Conjugate heat transfer is used to describe the processes which involves variations of temperature within the solids and fluids, due to thermal interaction between the solids and fluids.
The CHT analysis is generally used in places where there is heat transfer taking place for example when a fluid flows through a pipe and the heat of the fluid is transfered to the pipe by convection. CHT analysis is very much useful in analysing the areas where there are two bodies in which heat transfer takes place. when this analysis is used it is easy to calculate the heat transfer coefficient temperature and velocity of a fluid flowing through a certain body.
This type of heat transfer corresponds to the combination of the heat transfer in solids and heat transfer in fluids. heat transfer in solid called as conduction and heat transfer in fluid called convection.
conduction occurs due to the direct molecular collison. the faster kinetic energy collides with the low speed kinetic energy and the heat tranfer occur.
convection process is not happed alone. it is combined with the conduction or radiation. it occurs mostly in the cooling process like Heat sink, Battery cooling, Airflow over the solid object for cooling.
whenever the solid and fluid interface appears, this type of CHT analysis can be done to find the optimum result by using Ansys. It allows the simulation of heat transfer between solid and fluid domains by exchanging the thermal energy at the interface between them.
2. Geometry:
Prepare - Volume Extract - Share Topology
BASIC MESHING:
no. of Elements: 136143
nodes: 33243
Element size: 0.15m
Setup:
k-epsilon
velocity - 5m/s
Temperature - 700k
Convection - HTC - 20
150 iterations
Mesh:
Quality:
TEMPERATURE:
VELOCITY STREAM LINE:
VELOCITY PLANE:
WALL HEAT TRANSFER:
MESHING:
no. of Elements: 472482
body mesh -0.013m
Element size: 0.1m
Inflation
Quality:
TEMPERATURE:
VELOCITY STREAM LINE:
VELOCITY PLANE:
WALL HEAT TRANSFER:
Observations:
From both the cases it is identified that the temperature is low in case 2 compared to case 1 due to the addition of inflation layers and refined mesh. The wall heat transfer coeeficient is having major difference in the two cases. The flow is higher at the inlet as the Reynolds number is higher as well as the heat transfer is higher.
|
Parameters |
Basing Meshing |
Refined Meshing |
|
Temperature |
700.2k |
700k |
|
Velocity |
37.97m/s |
42.23m/s |
|
Wall Heat Transfer |
104.5[W m^-2 k^-1] |
224[W m^-2 k^-1] |
3. HTC PREDICTIONS:
Renoylds number is used to find the flow is laminar or tubulent. here, the natural cooling where Renolyds number depends on nusselts number.
For fully developed turbulent flow in a smooth circular tube, the local Nusselt number may be obtained from well known Dittus Boelter equation.
The Dittus Boelter equation may be used for small to moderate temperature differences
To Calculate nusselts number for turbulence model we need to Know Renolds number and Prandtl number.
Nu = 0.023(Re)^4/5 (Pr)^n
Nu = hl/k
Factors of Accuracy Depends:
Inflation Layers: Inflation is done to provide body fitted mesh to get accurate analysis of geometry.
Number of elements in meshing: Finer mesh gives more accurate results.
Topology: sharing topology helps in smooth transferring of conformal mesh in different zones.
Convergence: giving number of iterations to get converged.