Week 4 - CHT Analysis on Exhaust port

The important parameters, that are to be analysed for the successful design of any heat-transfer devices are temperature, heatflux and heat transfer coefficient of the surfaces exposed to heating or cooling environment. However accurate calculation of convective heat transfer coefficient requires boundary conditions consisting of the coupling of solid and fluidfields. Such a boundary condition assumes that the heat convected in or out to or from the solid is absorbed from or rejected by the fluid passing over the solid surface. To have an analytical or numerical prediction of heat transfer coefficient close to experimentally obtained values, one must focus on conjugating the boundary condition at the fluid–solid interface through coupled heat transfer analysis. Such a coupled field study is termed a Conjugate Heat Transfer (CHT) analysis. However, it may be noted that Coupled field analysis is not just limited to CHT studies involving fluid and solid domains. Coupled field analysis or specifically CHT analysis is essential in a situation where the solution domain consists of more than one sub-domains in which different differential equations govern heat transfer.

Conjugate heat transfer is a heat transfer analysis between solids and fluids. This type of heat transfer includes both convection (between fluids) and conductive (between solids) heat transfer and both forced and natural convection. In other words, conjugate heat transfer is a coupled multiphysics problem, and state variables in the solid domain (temperature T) and flow domain (pressure p and velocity v) are solved simultaneously in the flow simulation.

PROCEDURE

THE PURPOSE OF CONJUGATE HEAT TRANSFER ANALYSIS IS TO VARIOUS TEMPERATURES CONCERNING SOLIDS AND FLUIDS, DUE TO THERMAL INTERACTION BETWEEN THEM LIKE HEATING WATER IN A PAN OR HEAT TRANSFER FROM THE ENGINE TO THE RADIATOR AND RADIATOR TO ATMOSPHERIC AIR.

STEPS TO BE FOLLOWED

Step 1: Initially, start with importing the geometry to the Ansys workbench

If you check the geometry it has solid volume so we need to extract the fluid volume by following prepare -> volume extract -> select edge selection -> then select all the edges (inlet and outlet ).

Step 2: From the geometry create two kinds of mesh (coarse mesh and fine mesh) to perform grid independent study

1 coarse mesh

2 Fine mesh

The fine mesh has been created with an inflation layer according to the turbulence model and refined the mesh size of fluid volume has to get a more accurate result

After generating the mesh, the number of nodes and elements is investigated using the statistics option. Then the mesh quality is checked by choosing the mesh matrices option in the quality section.

Step 3:In this step, the meshed geometric file has been imported to the fluent module and set the following conditions as follows in the fluent in solving the problem.

This procedure is followed in Ansys fluent

SOLVER TYPE	Pressure based
VELOCITY FORMULATION	Absolute
TIME	Steady
VISCOUS MODEL	K-omega SST model and enable energy equation to solve the energy equation
MATERIAL	Fluid: air Solid: aluminium
CELL ZONES	Fluid flow region: air Solid region: Aluminium
BOUNDARIES (depends on the momentum ratio)	Inlet (velocity inlet)=5m/s Temperature=700k Outlet(pressure outlet) Internal solid wall=wall Outer wall=wall convection 20 `w/(m^2k)`
Report definition	Area weighted average wall flex Surface heat transfer coefficient on the internal pipe wall

the hybrid scheme is used for the initialization

Results for coarse mesh

contour plots

Velocity contour

Temperature contour

Streamline plot

Heat transfer coefficient

Results for fine mesh

contour plots

Velocity contour

Temperature contour

Streamline plot

• By observing the velocity plot it will be apparent that velocity is maximum at the outlet than the inlet because the mass outlet is 4 times the mass of the inlet (due to 4 inlets and 1 outlet) and the contour shows velocity is varying while fluid is approaching from inlet pipe to outlet pipe.
• the heat transfer coefficient is high in the outlet pipe than in the inlet pipe, So that's the reason for the high temperature in the outlet pipe than in the inlet (the high surface temperature at the outlet happens because of the all pipe meeting together)

Verifying the HTC predictions

1. According to Bernoulli’s principle in an incompressible flow mass entering the region is equal to the mass leaving the region in other words Bernoulli’s principle, states that the whole mechanical energy of the moving fluid, which includes the gravitational potential energy of elevation, fluid pressure energy, and kinetic energy of fluid motion, remains constant.
   As mass is neither created nor destroyed, mass flow rates are also balanced
   `Delta m_1+Delta m_2+ Delta m_3 +Delta m_4 =Delta m_(out)`
   `Delta m=rho Av Deltat`
   The mass flow will be calculated for unit time and the computed results are shown below
   
2. The Nusselt number is the ratio of convective to conductive heat transfer across a boundary.
   `Nu_L=" convective heat transfer"/"conductive heat transfer"=(hL)/k`
   Nusselt number is related to the Reynolds number
   `Nu=0.023 Re_D^(4/5) Pr^n`
   `Re=(rho u l)/mu`From the above equation, it will be clear that heat transfer is directly proportional to the Reynolds number so by changing the velocity of fluid the heat transfer changes with respect to it.