Exhaust Port Challenge

Aim

To conduct CHT(conjugate heat transfer) analysis over Exhaust port using CFD simulation and observe the following:

• Wall heat transfer coefficient on the internal solid surface
• Velocity contours
• Temperature contours
• Highest temperature location and reason

Introduction

An Exhaust manifold is one of the critical components of the IC engine for improving the volumetric efficiency by reducing the backpressure. The exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. This work considers 4 cylinders inline engine exhaust manifold and analyzes the flow of hot exhaust gas through the exhaust manifold and how the gas heats manifold i.e. the temperature variation, also confirming the wall heat transfer coefficient variation using velocity contour using CFD simulation in ANSYS FLUENT.

Conjugate Heat Transfer(CHT)

The term CHT is used to describe processes that involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids therefore these simulations are referred to coupled fluid-solid temperature calculations. In this approach, the mutual effects of thermal conduction in the solid and convection in the fluid are considered in the analysis. Considering the combined effect of heat transfer in both fluids and solids is a key to effectively design coolers, heaters, or heat exchangers. Heat transfer is mostly combined in solids and fluids as fluid flows around or between solid walls or else solids are immersed in a fluid.

Some problems that are being solved using CHT

• Calculating overall heat transfer coefficient eg. car radiator
• Extreme temperature in a system eg. in a gas turbine, chemical reaction vessels, food ovens
• Temperature distribution (related to thermal stress) eg. spacecraft walls
• Temperature response in time-dependent heating/cooling problems eg. engine cooling, or how fast car heat up in the sun and how is it affected by the shape of the windshield
• Motor and battery CHT analysis
• CHT analysis output (temperature distribution in solid and coolant of an engine, velocity distribution, mechanism of heat transfer) becomes the input to structural simulations as the thermal load

Geometry

Solid + Fluid volume

Exhaust manifold of 4 cylinders inline engine showing the connection between 4 Inlets and 1 outlet

Extracted flow volume

solid exhaust manifold without fluid volume

Name selections

4 inlets, 1 outlet, the outer wall of exhaust manifold as outer wall convection

the inner wall is named as inflation layers will be used to provide inflation layer at solid & fluid interface

Mesh

First simulation setup was confirmed using baseline mesh after that mesh was refined by reducing element size in body sizing of solid part and adding inflation layer at inner surface to capture the body geometry as we have to calculate wall heat transfer coefficient for which we need a good mesh near the fluid-solid interface.

Baseline Mesh

Default mesh is generated

Element size = 150mm

Elements = 137216

Nodes= 27373

Refined Mesh

Body sizing-

Geometry = solid body

Element size = 16mm

Inflation layer

Geometry - all faces in chosen named selections

Named selection = inflation layers

Total thickness - max thickness = 5mm

number of layers = 5

growth rate = 1.2

Elements = 461954

Nodes= 162542

Mesh generated

Inflation layer at the solid-fluid interface

Boundary Conditions

Inlet - velocity inlet: 5m/s @ 700k

outer wall convection - thermal - convection - heat transfer coefficient= 20 W/m^2/k (assumed/ from experiments)

Outlet- Pressure outlet: 0 gauge pressure

other boundaries whose names are not specified are by default set to the adiabatic wall (zero heat flux)

Solution

Pressure based solver

Steady-state

The energy equation is turned on as temperature as output is needed

Material - Fluid: Air

Solid: Aluminium

Turbulence: k-epsilon model

Hybrid Initialisation

Results

Temperature contour is defined for all boundaries

Animation file is created to see the temperature contour variation

Contours are plotted in an XY plane passing through the origin

Baseline Mesh

The temperature at the outer wall

Velocity streamline

Wall heat transfer coefficient

Here for baseline setup irregular mesh is present therefore not getting a smooth contour

Using Baseline mesh setup we confirm that our setup is correct and providing expected result pattern though values may be incorrect. Now using refined mesh we can achieve correct results.

Refined Mesh

Pressure variation

Exhaust manifold maintains such a pressure difference which helps to remove the exhaust gases. As fluid flows from high pressure to low pressure region.

The temperature at the outer wall

Despite inlet fluid temperature input being the highest (700k), the max temperature at the outer wall is near the exhaust pipe area

Velocity streamlines

Observing the velocity streamlines, High velocity can be observed at the same area where the temperature was also high. High velocity can be explained by mass conservation as 4 inlets and only 1 outlet. Therefore to conserve mass velocity at the exhaust area increases.

Velocity contour near the exhaust

Heat transfer coefficient

Wall heat transfer coefficient is high near the outlet side

Maximum wall heat transfer coefficient can be seen at the same location where temperature and velocity are maximum

As the wall heat transfer coefficient is calculated at the first cell thickness only which is near to the wall.

As it can be seen, we get a smooth contour at the inner wall due to inflation layer which follows the geometry and an irregular pattern at the outer wall due to irregular mesh

Wall heat transfer coefficient variation at the inner wall near the region of highest temperature and velocity

Conclusion

• Velocity is highest at the outlet section due to mass conservation as there are 4 inlets and only 1 outlet so to compensate the mass flow rate velocity is increased near the outlet.
• High velocity means high Reynolds number as it is directly proportional to the velocity.
• Nusselt number is directly dependent on Reynolds number, therefore, Nusselt number also increases with Reynolds number
• The Wall heat transfer coefficient directly depends on the Nusselt number, so the heat transfer coefficient increases with an increase in Nusselt number.
• Therefore the wall heat transfer coefficient is high at the outlet region due to the high velocity of fluid there.
• Due to the high heat transfer coefficient, we get more heat transfer from the fluid to the solid wall/pipe at the outlet region
• So here we confirm the reason for getting a high temperature near the outlet region than compared to the inlet region which also verifies that the results shown are correct.
• For Verification, calculate Reynold number and Prandtl number from that calculate Nusselt number then from Nusselt number we Heat transfer coefficient
• The accuracy of the solution is confirmed by mesh refinement and a proper inflation layer.
• Fine mesh help capture the physics more accurately though taking more time to converge.