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CHAP 5 FINITE ELEMENTS FOR HEAT TRANSFER PROBLEMS

CHAP 5 FINITE ELEMENTS FOR HEAT TRANSFER PROBLEMS. FINITE ELEMENT ANALYSIS AND DESIGN Nam-Ho Kim Audio by Raphael Haftka. HEAT CONDUCTION ANALYSIS. Analogy between Stress and Heat Conduction Analysis

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CHAP 5 FINITE ELEMENTS FOR HEAT TRANSFER PROBLEMS

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  1. CHAP 5 FINITE ELEMENTS FOR HEAT TRANSFER PROBLEMS FINITE ELEMENT ANALYSIS AND DESIGN Nam-Ho Kim Audio by Raphael Haftka

  2. HEAT CONDUCTION ANALYSIS • Analogy between Stress and Heat Conduction Analysis • In finite element viewpoint, two problems are similar if a proper interpretation is given. Heat conduction is somewhat simpler because temperature is scalar. • More Complex Problems • Coupled structural-thermal problems (thermal strain). • Radiation problem

  3. THERMAL PROBLEM • Goals: • Solve for temperature distribution for a given thermal load. • Boundary Conditions • Essential BC: Specified temperature • Natural BC: Specified heat flux

  4. Thigh Tlow Thigh qx qx Tlow STEADY-STATE HEAT TRANSFER PROBLEM • Fourier Heat Conduction Equation: • Heat flow from high temperature to low temperature • Examples of 1D heat conduction problems Thermal conductivity (W/m/C ) Heat flux (Watts)

  5. Quiz-like questions • What makes solving for temperatures and heat fluxes simpler than solving for displacements and stresses? • What are essential boundary conditions for both types of problems, and why they are called essential? • To generate heat flux between two points of different temperatures, we connect them with a conducting element, such as copper or steel wire. For the same heat flux, which metal will have a higher temperature difference between the two points? • For copper k=400 W/(m oC). If the wire has a cross section of 1cm2, and a length of 1m, what temperature difference will we have if we have 10W streaming through the wire? • Answers in notes page

  6. Qs Qg A dx GOVERNING DIFFERENTIAL EQUATION • Conservation of Energy • Energy In + Energy Generated = Energy Out + Energy Increase • Two modes of heat transfer through the boundary • Prescribed surface heat flow Qs per unit area • Convective heat transfer • h: convection coefficient (W/m2/C )

  7. Qs Qg A dx GOVERNING DIFFERENTIAL EQUATION cont. • Conservation of Energy at Steady State • No change in internal energy (DU = 0) • P: perimeter of the cross-section • Apply Fourier Law • Rate of change of heat flux is equal to the sum of heat generated and heat transferred

  8. BOUNDARY CONDITIONS • Temperature at the boundary is prescribed (essential BC) • Heat flux is prescribed (natural BC) • Example: essential BC at x = 0, and natural BC at x = L:

  9. N 2 1 Q1 Q2 QN Q3 j i e L(e) Tj Ti xi xj DIRECT METHOD HEAT CONDUCTION • Follow the same procedure with 1D bar element • No need to use differential equation • Element conduction equation • Heat can enter the system only through the nodes • Qi: heat enters at node i (Watts) • Divide the solid into a number of elements • Each element has two nodes and two DOFs (Ti and Tj) • For each element, heat entering the element is positive

  10. ELEMENT EQUATION • Fourier law of heat conduction • From the conservation of energy for the element • Combine the two equation • Similar to 1D bar element (k = E, T = u, q = f) Element conductance matrix

  11. Q2 Element 2 Element 1 2 1 3 ASSEMBLY • Assembly using heat conservation at nodes • Remember that heat flow into the element is positive • Equilibrium of heat flow: • Same assembly procedure with 1D bar elements • Applying BC • Striking-the-rows works, but not striking-the-columns because prescribed temperatures are not usually zero

  12. Q4 = –200W T1 T2 T3 T4 T5 x 200 C 1 2 3 4 Q1 Q2 = 500W Q3 = 0 Q5 = 0 EXAMPLE 5.1 • Calculate nodal temperatures of four elements • A = 1m2, L = 1m, k = 10W/m/C • Element conduction equation What is Q1?

  13. EXAMPLE cont. • Assembly • Boundary conditions (T1 = 200 oC, Q1 is unknown)

  14. EXAMPLE cont. • Boundary conditions • Strike the first row • Instead of striking the first column, multiply the first column with T1 = 200 oC and move to RHS • Now, the global matrix is positive-definite and can be solved for nodal temperatures

  15. EXAMPLE concluded • Nodal temperatures • How much heat input is required to maintain T1 = 200oC? • Use the deleted first row with known nodal temperatures

  16. Quiz-like problem • Consider the two element model in the figure. For both elements A = 100cm2, L = 1m, and the conduction coefficients are k1=100W/m/C, k2= 200W/m/C • Calculate the nodal temperatures and the heat flux Q1. • Answer on notes page

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