Week 4 - CHT Analysis on Exhaust port

AIM :

1. Give a brief description of why and where a CHT analysis is used.
2. Maintain the y+ value according to the turbulence model and justify the results.
3. Calculate the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.
4. How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

THEORY :

What is CHT Analysis, Why and Where it is used?

Conjugate Heat Transfer (CHT) refers to the coupling of conduction in solids with the convective and radiative heat transfer in the surrounding fluids. In other words, CHT corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective heaters, coolers, or heat exchangers. Heat transfer in solids and heat transfer in fluids are combined in majority of applications. This is because fluids flow around the solids, or between solid walls, and because solids are usually immersed in a fluid.

The CHT Analysis is used in processes which involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids. The exchange of thermal energy between the two physical bodies is called study of Heat Transfer, the rate of transferred heat is directly proportional to the temperature difference between the bodies. A typical example is the heating or cooling of a solid object by the flow of air in which it is immersed.

The physics of conjugate heat transfer is common in many engineering applications, including heat exchangers, HVAC, and electronic component design.

Application: The conjugate heat transfer methods have become a more powerful tool for modeling and investigating nature phenomena and engineering systems in different areas ranging from aerospace and nuclear reactors to thermal goods treatment and food processing from the complex procedures in medicines to ocean thermal interaction in metrology.

CHT in recent years has significantly improved the cooling performance of electronic equipment such as the design of heat sinks and the design of heat exchangers for the waste treatment plant. One such application of CHT is the exhaust port system.

SOLVING AND MODELLING APPROACH :

GEOMETRY -

DIMENSION -

IMPORT THE EXHAUST PORT SLDPRT MODEL IN ANSYS SPACE CLAIM.

CHANGE THE UNITS TO METERS FROM THE SPACECLAIM OPTIONS.

DELETE THE DUPLICATE EDGES BY REPAIR -> SELECT EXTRA EDGES. (IF THERE)

CREATE FLUID VOLUME TO REPRESENT THE FLUID FLOW BY THE FOLLOWING COMMAND. PREPARE --> CLICK ON VOLUME EXTRACT --> ACTIVATE EDGE SELECTOR AND SELECT ALL THE OPEN EDGES --> CLICK ON THE GREEN CHECKMARK.

TO SHARE THERMAL INFORMATION FROM FLUID TO SOLID, IT IS IMPORTANT TO GENERATE THE CONFORMAL MESH. THE MESH AT THE FLUID-SOLID INTERFACE SHOULD HAVE PROPER NODE CONNECTIVITY SO THAT A PROPER FLOW OF INFORMATION TAKES PLACE FROM FLUID TO SOLID. TO CREATE A CONFORMAL MESH, ACTIVATE SHARE TOPOLOGY. THIS CAN BE DONE FFF1 -> ANALYSIS -> SHARE TOPOLOGY -> SHARE.

TO SHARE INFORMATION AND TO CREATE AN INTERFACE ACCURATELY, GO TO WORKBENCH -> SHARE.

THE PINK COLOR INDICATES THE INTERFACE IS GENERATED B/W FLUID AND SOLID VOLUMES AND PROPER FLOW OF INFORMATION TAKES PLACE.

MESH -

NOTE - BEFORE GENERATING THE MESH, CREATE NAMED SELECTION.

THE NAMED SELECTIONS ARE AS FOLLOWS, INLET, OUTLET AND OUTER_WALL_CONVECTION.

FIRST, GENERATE A MESH OF ELEMENT SIZE - 150MM THROUGHOUT THE DOMAIN.

GENERATE FINER MESH IN THE SOLID VOLUME OF ELEMENT SIZE 0.1M BY INSERTING BODY SIZING IN THE SOLID VOLUME.

TO CAPTURE WALL EFFECTS ACCURATELY AT THE INTERFACE, GENERATE INFLATION LAYERS.

NO. OF ELEMENTS = 240308 AND NO. OF NODES = 85385.

UPDATE THE MESH AND START THE SETUP.

# CASE - USING K EPSILON METHOD

FLUENT SETUP :

ONCE THE MESHING PART IS COMPLETED, CLOSE THE MESH AND UPDATE THE MESH.

OPEN THE "SETUP". SET THE SOLVER TYPE AS PRESSURE BASED

UNDER MODELS ENABLE THE ENERGY EQUATION AND OPEN VISCOUS SELECT THE K-EPSILON MODEL AND CLICK OK.

THEN UNDER MATERIAL SET THE FLUID TO BE AIR AND SOLID TO BE ALUMINUM

UNDER CELL ZONE CONDITIONS MAKE SURE THAT THE SELECTED FLUID IS AIR AND THE SOLID IS ALUMINUM.

SELECT THE BOUNDARY CONDITIONS PARAMETER AND SET THE BOUNDARY CONDITIONS FOR THE CREATED NAMED SELECTIONS.

BOUNDARY CONDITIONS -

1) INLET - TYPE: VELOCITY INLET

V = 5M/S, T = 700K

2) OUTLET - TYPE: PRESSURE OUTLET

PGAUGE = 0 PA

3) OUTER-WALLS-CONVECTION - TYPE: WALLS

SELECT CONVECTION

H = 20 W/M2-K

4) WALL-FINAL_GEOMETRY-SOLID_VOLUME - TYPE: WALLS

ADIABATIC BOUNDARY CONDITIONS

DEFINATIONS -

SELECT NEW - SURFACE REPORT- AREA WEIGHTED AVERAGE PLOT

HEAT TRANSFER COEFFICIENT IS COMPUTED BY USING SURFACE AVERAGE METHOD.

INITIALIZE THE MODEL BY USING HYBRID METHOD.

SIMULATION IS CARRIED OUT FOR A TOTAL OF 450 ITERATIONS.

RESULTS :

RESIDUALS -

TEMPERATURE CONTOUR -

HEAT TRANSFER COEFFICIENT -

VELOCITY STREAMLINE -

WALL HEAT TRANSFER COEFFICIENT -

VELOCITY PLANE-

TEMPERATURE PLANE -

ANALYTICAL HEAT TRANSFER CO-EFFICIENT CALCULATION:

AT INLET:

IN THIS MODEL, SINCE WORKING FLUID IS BEING COOLED IN THE EXHAUST MANIFOLD, N IS TAKEN AS 0.3

THE CHARACTERISTIC LENGTH FOR THE EXHAUST PORT IS 150MM(DIAMETER)

PROPERTIES OF WORKING FLUID(AIR) -

DENSITY = 1.225 KG/M3, THERMAL CONDUCTIVITY = 0.0242 W/M-K, DYNAMIC VISCOSIT = 1.789E-5 PA-S, SPECIFIC HEAT AT CONSTANT PRESSURE = 1006.43 J/KG-K

FROM THE ABOVE EQUATION, WE GET

(HD/K) = 0.023*(V*D/Ν)^0.8*(Ν/Α)^0.3

H = 19.92 W/M2-K

CONCLUSION :

• THE CHT ANALYSIS ON THE EXHAUST PORT WAS SIMULATED SUCCESSFULLY ON ANSYS-FLUENT.
• IMPORTING THE EXHAUST PORT GEOMETRY IN SPACECLAIM AND NAMED ALL THE SELECTION PART
• MESH GEOMETRY ADD INFLATION LAYER
• NO. OF ELEMENTS = 240308 AND NO. OF NODES = 85385.
• ON FLUENT SELECT K EPSILON MODEL , BOUNDARIES , ENERGY , CONTOURS , ANIMATION ETC AND HYBRID INITILIZE
• CALCULATE ADDING 450 NO. OF ITERATIONS
• RESULTS OUT ALL THE REQUIRED PLOTS OF TEMPERATURE, VELOCITY, STREAMLINE VELOCITY ETC
• BECAUSE OF INFLATION LAYER THE RESULT IS MORE SMOOTH.
• THE ANALYTICAL VALUE OF THE HEAT TRANSFER COEFFICIENT IS 19.81 WM2.K
• THE DEVIATION OF NUMERICAL HEAT TRANSFER COEFFICIENT VALUES FROM THE ANALYTICAL SOLUTION WAS FOUND TO BE WITHIN 5% TO THE CASE