Week 6 - CHT Analysis on a Graphics card

Aim:To Perform a steady-state conjugate heat transfer analysis on a model of a graphics card.

Introduction& theory:

The Conjugate Heat Transfer (CHT) analysis type allows for the simulation of heat transferbetween solid and fluid domains by exchanging thermal energy at the interfaces between them This type of heat transfer includes both convection (between fluids) and conductive (between solids) heat transfer, as well as both forced and natural convection.

Forced convection:

Convection is the mechanism of heat transfer through a fluid in the presence of bulk fluidmotion. Convection is classified as natural (or free) and forced convection depending on how the fluid motion is initiated. In natural convection, any fluid motion is caused bynatural means such as the buoyancy effect, Whereas in forced convection, the fluid is forced to flow over a surface or in a tube by external means such as a pump or fan

Convection heat transfer is complicated since it involves fluid motion as well as heat conduction. The fluid motion enhances heat transfer (the higher the velocity the higher the heat transfer rate).The rate of convection heat transfer is expressed by Newton’s law of cooling,

Q=hA(T2-T1).

The convective heat transfer coefficient h strongly depends on the fluid properties and roughness of the solid surface, and the type of the fluid flow (laminar or turbulent)It is assumed that the velocity of the fluid is zero at the wall, this assumption is called no‐slip condition. As a result, the heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction, since the fluid is motionless.

About Heat:

Energy transfer to & from a open system in two distinct forms

1.Heat

2.Work

• Both heat & work recognized at the boundary of the system ie., at the walls as they cross boundaries.
• Heat & work are boundary phenomenon
• Both are associated with the process not a state
• Both are path functions ie,.there magnitudes depend on the path followed during a process
• Path functions have inexact diffentials .

Energy transfer b/w the system & surroundings on the temp diffie., from higher temp to lower temp until it attains thermal equillibrium is established.

An Energy interaction is heat only ,it takes place because of temp diff,no heat transfer takes place when both system & surroundings attains same temp/equillibrium.

Temp is the driving Force for heat transfer ,the larger temp diff ,the higher rate of heat transfer, once it transfer to surroundings the transfer heat becomes part of Internal energy of the surroundings.

An Energy interaction that is not caused by a temp diff b/w system & surroundings is work.Work is the energy transfer associated with force acting trough a distance during adiabatic process.

Heat & work are directional quantities ,thus complete description of heat & work requires specification of both Magnitude & direction.

Adiabatic Means:

• Either the system is well insulated so that neglizable amt of heat can pass.
• Both system & surroundings are at the same temp thermal equillibrium and there is no temp diff ie., driving force.
• Even though there is no heat transfer during adiabatic process, the Energy content /temp of the system can still be changed by other means such as work.

Modelling:

Geometry _

• Right click_Open_geometry_Editgeometry
• Geometry_workbench_share topology

Meshing:

• Meshing is done with 1.9 mm of enclosure & used Edge sizing (0.8) for small components like processor etc.,
• Mesh_rightclick_generate mesh
• Mesh_rightclick_insert_sizing(edges,body,faces)

Fluent:

• Open fluent_physics_energy eq on
• Fluent_physics_viscous_k-epsilon_ok
• Fluent_physics_materials_fluid _air
• Fluent_physics_materials_solid_aluminium_finsboard
• fluent_physics_materials_solid_silicon_processor
• fluent_physics_materials_solid_pcb_pcbboard
• fluent_physics_zones_assign _aluminium(fins),silicon(processor),pcb(pcbboard)
• fluent_physics _boundaryconditions_velocity,temp

For Case1:

Velocity=3m/sec.

Temp=300k.

Fluent_physics_energy eq_

Fluent_materials_creating_fluid & solid materials

Name Material Thermal conductivity Specific_heat Density

Fins aluminium 202.4 871 2719

Processor silicon 153 703 2330

Pcb_board polysterene 0.027 1210 55

Heat transfer of fluid air 0.0242 1006.43 1.225

Fluent_boundaries_inlet velocity_3m/sec

For Case2:

Velocity=1m/sec

Temp=300k.

Fluent_physics_energy eq_

Fluent_materials_creating_fluid & solid materials

Name Material Thermal conductivity Specific_heat Density

Fins aluminium 202.4 871 2719

processor silicon 153 703 2330

pcb_board polysterene 0.027 1210 55

Heat transfer of fluid air 0.0242 1006.43 1.225

Fluent_boundaries_inlet velocity_1m/sec

• heat_transfer_coeff___1.4540897e-11

• temp____303.82575

For Case3:

Velocity=5m/sec

Temp=300k.

Fluent_physics_energy eq_

Fluent_materials_creating_fluid & solid materials

Name Material Thermal conductivity Specific_heat Density

Fins aluminium 202.4 871 2719

processor silicon 153 703 2330

pcb_board polysterene 0.027 1210 55

Heat transfer of fluid air 0.0242 1006.43 1.225

Fluent_boundaries_inlet velocity_5m/sec

• heat_transfer_coeff___-1.7625818e-07

• temp___301.15985

My observation:

For velocity 3m/sec & temp300k

For velocity 5m/sec &300k

For velocity of 1m/sec & 300k

My observation:

Here Energy developed is Processor ..so Processor is the source of energy ..In order to give an cooling effect to the processor,the air is passed which gives an air cooling effect .the air is passed with different or varying velocities (3,1,5 m/sec)..the temp of the processor gradually decreases to some extent. the heat generated by processor will reject heat to the air ie.,which flow over.because of high temp,according to the first law of thermodynamics heat transfer takes place b/w two bodies from Higher temp to the Low temp untill it attains thermal equillibrium.The above graphs briefly explains the for given rate of velocities the temp change..

we clearly observe that slight change(rejection) in the temp for different velocities its for 1m/sec..363k & for 3m/sec its 333k & for 5m/sec its 325k...by these values clearly explains the a slight change in the heat.so if we gradually increase in velocities the temp gradually decreases.

case1(3m/sec) case2 (1m/sec) case3 (5m/sec)

• Heat_transfer_coeff=-7.3907221e-08 1.4597959e-11 -1.7625818e-07

  temp__ 301.67641 303.82575 301.15985

The above avg heat_transfer_coeffcient & avg temp is observed.

My conclusion:

• Here as rate of velocity increases the rate of temp decreases as we discuss in the above.