Conjuagate Heat Transfer CHT analysis on an exhaust manifold

1. Give a brief description of why and where a CHT analysis is used.

Ans:

  • CONJUGATE HEAT TRANSFER(CHT) refers to situations in which multiple modes of heat transfer occur simultaneously.
  • Simultaneous multiple heat transfer modes occur in almost all real-world problems, So CHT is quite important as majority of heat transfer applications involve more than one mode of heat transfer, whereas in some cases certain heat transfer modes can be neglected.
  • Conjugate heat transfer (CHT) allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them.
  • It requires a multiregion mesh to have a clear definition of the interfaces in the computational domain
  • Conjugate heat transfer(CHT) analysis is employed in desiging and simulating of heat transfer appliances like heat exchangers, cooling of electronic equipment, Thermal insulators, Heat sink, Exhaust port manifold and general-purpose cooling and heating systems.

Geometry of Exhaust Manifold:

           Fig: Exhaust port manifold for 4 cylinder in -line IC Engine

Baseline Mesh for Exhaust port Manifold:

Element size = 150

Number of Nodes = 27411

Number of Elements =137445

Simulation Details:

Type of simulation = steady state

Type of Model = k-epsilon(Turbulence Model)

Working fluid = Air

Solid material = Aluminium

Inlet Velocity = 5 m/s

Inlet temperature = 700 K

 

Outer wall heat transfer coefficient = `20 W/m^2 K`

Scaled Residuals: 

Contours of static temperature:

Temperature:

Stream lines velocity:

Velocity:

Wall Heat transfer coefficient:

Refined Mesh for Exhaust port Manifold:

Element size = 150

Body Element size = 16

Number of Nodes = 162574

Number of Elements =462316

Scaled Residuals: 

Contours of static temperature:

Temperature:

Stream lines velocity:

Velocity:

Wall Heat transfer coefficient:

 

3. How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on

Heat Transfer coefficient at the boundary is calcuated using a Dittus-Boelter correlation.

                    `Nu = 0.023*(Re)^(4/5) * pr^n`

                          n = 0.4 for heating of fluids

                          n = 0.3 for cooling of fluids

                    Nusselt number 

                           `Nu = {hD}/k`

                          D = characteristic Dimension 

                          k = Thermal conducutivity of fluid         

 

  • The temperature results of both the baseline mesh and refined mesh are almost same, there is no predominant variation within the contours.
  • Refined mesh takes more time to converge as the number of cell count increases. If the mesh is finer, it is ensured that the curvature and other geometrical features are properly captured.
  • Because of the inflation layers, the quality of the mesh near the boundary interface increases and it captures smooth plots of heat transfer coefficient  
  • The Wall heat transfer coefficient obtained from the simulation is compared with the wall heat transfer coefficient which is obtained by a Dittus-Boelter correlation. 
  • If the numerical simulation results and Analytical results are closer than we can conclude that the Heat transfer coefficient obtained from the simulation results is accurate.

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