Chapter 11 Thermal Contact

Thermal Contact Contact Resistance DT Two surfaces (at different temperatures) in contact experience a temperature drop across the interface. The drop is due to imperfect contact between the two surfaces. The imperfect contact, and hence the contact resistance, can be influenced by many factors such as: surface flatness surface finish oxides entrapped fluids contact pressure surface temperature use of conductive grease DT T x Inventory #01445 March 15, 2001 11-2

Thermal Contact ANSYS can simulate this behavior and other complex thermal problems using … Coupled-field structural-thermal Contact Supports general thermal contact analysis capability. Typical applications: Metal forming Assembly contact Gas turbine Thermal Assembly Frictional Heating Inventory #01445 March 15, 2001 11-3

Thermal Contact Coupled-field solids (PLANE13, SOLID5) and surface-to-surface contact elements with KEYOPT(1) = 1 Can also be used with pure thermal elements Fix all structural DOFs on contact elements Key features: Heat conduction between contacting surfaces Heat generation due to frictional dissipated energy Heat convection and/or radiation Between surfaces with small gap From free surface to environment Heat flux input at open gap NOTE: This course will focus on conduction between contacting surfaces. See the ANSYS documentation for more information regarding additional thermal contact capabilities. Inventory #01445 March 15, 2001 11-4

Thermal Contact Heat conduction: q = TCC * (TT - TC) TCC is thermal contact conductance coefficient (real constant) Can be a table parameter (function of pressure and temperature) TT and TC are target and contact surface temperatures Heat flows when contact status is closed Model temperature discontinuity across contact interface No DT (continuous material) DT (contact interface) Inventory #01445 March 15, 2001 11-5

Thermal Contact Frictional heat generation: q = FHTG * t * v FHTG is fraction of energy converted to heat (real constant) t is the equivalent frictional stress v is the sliding rate Heat can be distributed unequally between contact and target: qc = FWGT * FHTG * t * v qt = (1-FWGT) * FHTG * t * v Frictional energy is typically dissipated in three forms: Acoustic (noise) Mechanical (abrasion) Thermal (heat) FHTG is the fraction of frictional energy dissipated as heat. By default, FHTG=1.0 By default, the frictional heat energy is evenly distributed between the contact and target surfaces (FWGT=0.5) Brake Pad on Wheel Inventory #01445 March 15, 2001 11-6

Thermal Contact Convection: q = CONV * (TE - TC) CONV is convection coefficient (SFE table parameter load) TE is target temperature, or bulk temp for free surface (SFE) TC is contact temperature Heat flows between contact and target when 0 < gap < pinball Heat flows from contact to environment for free surface Free surface is recognized for any of these conditions: Open far-field contact (gap > pinball) Contact elements only (omit target elements) If target elements are present, Keyopt(3)=1 for target elements Inventory #01445 March 15, 2001 11-7

Thermal Contact Radiation: q = RDVF * EMIS * SBCT * [(TE + TOFFST)4 - (TC + TOFFST)4] RDVF is radiation view factor (real constant) RDVF can be table parameter (function of time, temp, gap distance) EMIS is surface emissivity (material property) SBCT is Stefan-Boltzmann constant (real constant) TOFFST is temperature offset from absolute zero (TOFFST command) Heat flows between contact and target when 0 < gap < pinball Heat flows from contact to environment for free surface Free surface recognized as for convection Inventory #01445 March 15, 2001 11-8

Thermal Contact External heat flux input SFE applied to contact surface only (not target surface) Heat flux acts only if contact status is open For near-field contact, the flux acts on both contact and target For free surface flux acts only on contact element Free surface recognized as for convection Cannot be applied simultaneously with convection on a given element Inventory #01445 March 15, 2001 11-9

Thermal Contact Thermal contact tips Conductance TCC Has units of heat / (time*degree*area) Typically less than the equivalent conductance of contacting solids For frictional heating, TIME must have true chronological units However, if structural inertia and damping are unimportant, turn them off with TIMINT,STRUC,OFF for faster solution Unsymmetric solver option may benefit frictional sliding NROPT,UNSYM Inventory #01445 March 15, 2001 11-10

Thermal Contact … Example Problem Thermal Contact – Conductance Problem Two bodies of dissimilar materials are in contact. The extreme ends of the bodies are held at different, constant temperatures. A force on the top surface causes contact pressure between the two components. Inventory #01445 March 15, 2001 11-11

Thermal Contact … Example Problem This example will be solved as a plane strain, symmetric model. The top surface will be deflected 0.01 inches toward the bottom component. Contact elements will include constant thermal conductance. Inventory #01445 March 15, 2001 11-12

Thermal Contact … Example Problem Read in the model using the input file “th_contact.inp”. Create the contact pair using the Contact Wizard. Inventory #01445 March 15, 2001 11-13

Thermal Contact … Example Problem Select the top line of the bottom component for the Target Surface. Inventory #01445 March 15, 2001 11-14

Thermal Contact … Example Problem Select the bottom two lines of the top component for the Contact Surface. Inventory #01445 March 15, 2001 11-15

Thermal Contact … Example Problem Complete the creation of the contact pair with default options. Inventory #01445 March 15, 2001 11-16

Thermal Contact … Example Problem Modify the Contact Element options to include the TEMP degree of freedom. Inventory #01445 March 15, 2001 11-17

Thermal Contact … Example Problem Modify the Contact Element REAL constant to include the thermal contact conductance coefficient (TCC). Inventory #01445 March 15, 2001 11-18

Thermal Contact … Example Problem For this example, use a constant TCC = .001. Thermal Contact Options Inventory #01445 March 15, 2001 11-19

Thermal Contact … Example Problem Apply the structural boundary conditions. Symmetry Fixed in X & Y Inventory #01445 March 15, 2001 11-20

Thermal Contact … Example Problem Apply the enforced displacement. Displaced -0.01 in Y-dir Inventory #01445 March 15, 2001 11-21

Thermal Contact … Example Problem Apply the thermal boundary conditions. Temp = 200 F Temp = 70 F Inventory #01445 March 15, 2001 11-22

Thermal Contact … Example Problem Run the solution. Plot deflection in the Y-Direction to verify structural solution. Inventory #01445 March 15, 2001 11-23

Thermal Contact … Example Problem Plot nodal TEMP. Note the mismatch in the temperature at the contact surface. Remember: TCC = .001. Inventory #01445 March 15, 2001 11-24

Thermal Contact … Example Problem Now re-run the problem after modifying TCC = 1. Inventory #01445 March 15, 2001 11-25

Thermal Contact … Example Problem Plot nodal TEMP. Note the consistent temperature at the contact surface. Remember: TCC = 1. Inventory #01445 March 15, 2001 11-26