Radiation Heat Transfer Experiment

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1 Radiation Heat Transfer Experiment Thermal Network Solution with TNSolver Bob Cochran Applied Computational Heat Transfer Seattle, WA ME 331 Introduction to Heat Transfer University of Washington November 18, 2014

2 2 / 43 Outline Math Model Geometry Test Data Thermal Network Model Solution with TNSolver Results

3 3 / 43 Surface Properties for Radiation Heat Transfer Math Model All surfaces emit thermal radiation. The emittance, ɛ, is the ratio of actual energy emitted to that of a black surface at the same temperature. All radiation impinging on a surface will either be reflected, absorbed or transmitted. Reflectance or reflectivity, ρ, is the amount reflected. Absorptance or absorptivity, α, is the amount absorbed. Transmittance or transmissivity, τ, is the amount transmitted through the material.

4 4 / 43 Surface Properties for Radiation Heat Transfer (continued) Math Model Summation property for all incident radiation: ρ + α + τ = 1 An opaque surface has τ = 0, so ρ + α = 0. Kirchoff s law provides that ɛ = α for gray, diffuse surfaces. Diffuse is a modifier which means the property is not a function of direction. Gray is a modifier indicating no dependence on wavelength. Spectral is a modifier which means dependence on wavelength. Specular is a modifier which means mirror-like reflection.

5 Characteristics of Real Surfaces Math Model Figure borrowed from M.F. Modest, Radiative Heat Transfer, second edition, Academic Press, New York, / 43

6 6 / 43 View Factor Properties Math Model Summation Rule (Equation (13.4), page 830 in [BLID11]): N F ij = 1 i=1 Reciprocity Rule (Equation (13.3), page 829 in [BLID11]): A i F ij = A j F ji Addition of View Factors for Subdivided Surfaces (Equation (13.5), page 833 and Figure 13.7, page 835 in [BLID11]): F i(j) = N k=1 F ik

7 7 / 43 View Factor for Coaxial Parallel Disks Math Model A j R i = r i L, R j = r j L r j S = R2 j R 2 i L F ij = 1 2 S S 2 4 ( rj r i ) 2 A i r i See Table 13.2, page 833 in [BLID11] or Table 10.3, page 546 in [LL12] Also see the web site: A Catalog of Radiation Heat Transfer Configuration Factors, by John R. Howell, specifically C-41: Disk to parallel coaxial disk of unequal radius.

8 8 / 43 View Factor for Conical Frustum Wall to its Base Math Model r t H = h r b R = r b r t X = 1 + R 2 + H 2 F wb = 2R2 X + X 2 4R 2 2 (1 + R) X 2R A w A b r b h See the web site: A Catalog of Radiation Heat Transfer Configuration Factors, by John R. Howell, specifically C-112: Interior of frustum of right circular cone to base.

9 9 / 43 View Factor for Disk to Coaxial Cone Math Model α = tan 1 r c h S = r d R = r c X = S + R cot α L r d A = X 2 + (1 + R) 2 B = X 2 + (1 R) 2 C = cos α + S sin α D = cos α S sin α E = R cot α S See the web site: A Catalog of Radiation Heat Transfer Configuration Factors, by John R. Howell, specifically C-48: Disk to coaxial cone. A c A d r d α r c L h

10 10 / 43 View Factor for Disk to Coaxial Cone (continued) Math Model For α tan 1 1 S : F dc = 1 2 For α < tan 1 1 S : F dc = 1 π + sin α cos 2 α { } R 2 + X (1 + R 2 + X 2 ) 2 4R 2 { 1 AC ABtan [ 1 CD XEtan X (1 BD + + S 2) tan 1 C D ( + S2 1 CD tan S + (CD)2 tan 1 X CD S )] tan 1 CD [ ] } R (X + S) + SR tan α cos 1 ( S tan α) sin 2α

11 11 / 43 Net Radiation Method for Enclosures The enclosure geometry is approximated by a set of N ideal surfaces Diffuse gray emission/absorption Diffuse gray reflections Each surface is isothermal, with uniform heat flux

12 12 / 43 Oppenheim Network for Three Surfaces Math Model 3 1 ɛ 3 ɛ 3A 3 J 3 1 A 2F 23 J 2 1 ɛ 2 ɛ 2A 2 2 e b3 e b2 1 1 A 1F 13 A 1F 12 J 1 1 ɛ 1 ɛ 1A 1 e b1 1 A.K. Oppenheim, Radiation Analysis by the Network Method, Transactions of the ASME, vol. 78, pp , 1956

13 13 / 43 The Surface Radiosity Math Model Radiosity, J i, for surface i is: Surface irradiation, G i is: J i = ɛ i σti 4 }{{} + ρ i G }{{} i emmision reflected irradiation G i = N F ij J j i=1 Then noting that ρ i = 1 ɛ i, the equation for the radiosity from surface i is: ( N ) J i (1 ɛ i ) F ij J j i=1 = ɛ i σt 4 i

14 System of Equations for the Radiosity Math Model Rearrangement of the radiosity equation for surface i leads to: N i=1 [ δij (1 ɛ i ) F ij ] Jj = ɛ i σt 4 i Combining the radiosity equations for each surface in the enclosure leads to a system of equations: Where: [A] {J} = {b} A ij = δ ij (1 ɛ i ) F ij b i = ɛ i σt 4 i The surface temperature, T i, is provided by the solution of the energy conservation equation. 14 / 43

15 15 / 43 Application of Radiosity to Heat Conduction Equation Math Model Once the radiosity, J i is known for a surface, the irradiation is evaluated: G i = N F ij J j i=1 This is then used to evaluate the net radiant thermal energy for surface i: q i radiation = ɛ i σt 4 i α i G i This heat flux boundary condition is then applied to the energy conservation equation solution so that updated surface temperatures can be determined. Iteration between the energy equation and the radiosity is continued until convergence.

16 16 / 43 Matrix Form of the Radiosity Equation Math Model where, for N = 3: J 1 {J} = J 2 J 3 [ɛ] = [ρ] = N i=1 ɛ ɛ ɛ 3 [ δij (1 ɛ i ) F ij ] Jj = ɛ i σt 4 i ([I] [ρ][f ]) {J} = [ɛ] {E b } ρ ρ ρ 3 { σt 4} = [F] = σt 4 1 σt 4 2 σt 4 3 F 11 F 12 F 13 F 21 F 22 F 23 F 31 F 32 F 33 = [I] [ɛ] = = {E b} = [I] = E b1 E b2 E b ɛ ɛ ɛ 3

17 17 / 43 Enclosure Heat Flow from Radiosity Math Model ([I] [ρ][f]){j} = [ɛ]{e b } Using the definition of radiosity: {J} = [ɛ]{e b } + [ρ]{g} = [ɛ]{e b } + [ρ][f]{j} and the net surface heat flux (α = ɛ): {q} = [ɛ]{e b } [α]{g} = [ɛ]{e b } [ɛ][f]{j} then some algebraic manipulation leads to: ([I] [F][ρ])[ɛ] 1 {q} = ([I] [F]){E b } or, {q} = [ɛ]([i] [F][ρ]) 1 ([I] [F]){E b }

18 18 / 43 Exchange Factor, F Math Model The exchange factor concept is based on proposing that there is a parameter, F ij, based on surface properties and enclosure geometry, that determines the radiative heat exchange between two surfaces [Hot54, HS67] : Q ij = A i F ij σ(t 4 i T 4 j ) F is known by many names in the literature: script-f, gray body configuration factor, transfer factor and Hottel called it the over-all interchange factor. For an enclosure with N surfaces, the net heat flow rate, Q i, for surface i, is: Q i = N j=1 ( ) A i F ij σti 4 σtj 4 = N ( ) A i F ij Ebi E bj j=1

19 19 / 43 Exchange Factor, F, Properties Math Model Reciprocity: A i F ij = A j F ji Summation: N F ij = ɛ i j=1

20 20 / 43 Matrix Form of Enclosure Radiation with F Math Model where, for N = 3: [F ] = Q i = N ( ) A i F ij Ebi E bj j=1 {Q} = [A] ([ɛ] [F ]) {E b } {q} = [A] 1 {Q} = ([ɛ] [F ]) {E b } {Q} = Q 1 Q 2 Q 3 F 11 F 12 F 13 F 21 F 22 F 23 F 31 F 32 F 33 [A] = {E b } = A A A 3 σt 4 1 σt 4 2 σt 4 3 [ɛ] = ɛ ɛ ɛ 3

21 21 / 43 Exchange Factors, F, from View Factors, F Math Model The net heat flux using view factors, F ij, is: {q} = [ɛ]([i] [F][ρ]) 1 ([I] [F]){E b } The net heat flux using exchange factors, F ij, is: {q} = ([ɛ] [F ]) {E b } The two enclosure heat fluxes are equal, so equating gives: ([ɛ] [F ]) {E b } = [ɛ]([i] [F][ρ]) 1 ([I] [F]){E b } ([ɛ] [F ]) = [ɛ]([i] [F][ρ]) 1 ([I] [F]) ( ) [F ] = [ɛ] [I] ([I] [F][ρ]) 1 ([I] [F])

22 Radiation Heat Transfer Experiment Geometry 22 / 43

23 Heater Geometry 23 / 43

24 Shield Geometry 24 / 43

25 Target Geometry 25 / 43

26 26 / 43 Surface Emissivity Material Properties Geometry Emissivity properties from Table A.8 in [BLID11] Material ɛ Target: Alloy 1100 Aluminum, coated flat black 0.95 Target: 304 Stainless Steel Heater: Ceran glass 0.8* Shield: Galvanized steel 0.28 * To be confirmed See [TD70] and [Gil02] for emissivity data, for a variety of materials.

27 27 / 43 Measurements Test Data For each target material: Heater temperature Water evaporation rate Ambient air temperature Distance from target to heater Shield in place or not

28 28 / 43 Sample Data Set Test Data Aluminum, Painted Flat Black, 4 Above The Heater Heater Temperature was 525 C Water Temperature was 100 C

29 29 / 43 Heat Transfer Analysis Thermal Network Model Energy conservation: control volumes Identify and sketch out the control volumes Use the conductor analogy to represent energy transfer between the control volumes and energy generation or storage Conduction, convection, radiation, other? Capacitance Sources or sinks State assumptions and determine appropriate parameters for each conductor Geometry, material properties, etc. Which conductor(s)/source(s)/capacitance(s) are important to the required results? Sensitivity analysis What is missing from the model? - peer/expert review

30 30 / 43 Thermal Network Terminology Thermal Network Model Geometry Control Volume Volume property, V = dv V Node:, T node = T (x V i)dv Control Volume Surface Area property, A = da A Surface Node:, T surface node = T (x A i)da Material properties Conductors Conduction, convection, radiation Boundary conditions Boundary node:

31 31 / 43 Network Model Thermal Network Model water t-w env t-s target t-e s-e shield h-s h-t h-e env heater

32 32 / 43 Radiation Conductors Thermal Network Model Q ij = σf ij A i (T 4 i T 4 j ) Begin Conductors! label type nd_i nd_j script-f A h-t radiation heater target h-s radiation heater shield h-e radiation heater env t-s radiation target shield t-e radiation target env s-e radiation shield env End Conductors Note that h r is reported in the output file.

33 33 / 43 Linearization of Radiation Conductors Thermal Network Model The temperature is linearized using a two term Taylor series expansion about the previous iteration temperature, T : T 4 j T 4 i (T i ) 4 + (T i T i ) 4(T i ) 3 Ti 4 4(Ti ) 3 T i 3(T ( ) 4 ( ) ( Tj + T j Tj 4 ( ) 3Tj Tj 4 4 Tj 3( Tj i ) 4 The linearized form of the heat transfer rate is: [ ( ) 3Tj Q ij = σf ij A i 4(Ti ) 3 T i 3(Ti ) 4 4 Tj + 3( { ( Q ij = σf ij A i 4(Ti ) 3 T i 4 T j T j ) 4 ) 3 T j ) 4 ] ) { 3Tj ( } σf ij A i 3(Ti ) 4 3 T j ) 4 }

34 34 / 43 Using the Function disk2disk.m Thermal Network Model >> help disk2disk [F12, A1, F21, A2] = disk2disk(r1, r2, h) Description: View factor between two coaxial disks. Inputs: r1 = radius of upper disk r2 = radius of lower disk h = distance between the disks Outputs: F12 = view factor from upper disk to lower disk A1 = area of the upper disk F21 = viewfactor from lower disk to upper disk A2 = area of the lower disk Reference: A Catalog of Radiation Heat Transfer Configuration Factors, third edition, John R. Howell, The University of Texas at Austin

35 35 / 43 Using the Function disk2disk.m Thermal Network Model r 1 = m, r 2 = m and h = 0.2m >>[F12, A1, F21, A2] = disk2disk(0.0889, , 0.2) F12 = A1 = F21 = A2 =

36 36 / 43 Using the Function cone2base.m Thermal Network Model >> help cone2base [F12, A1, F21, A2] = cone2base(rb, rt, h) Description: View factor between the interior wall of a conical frustum and its base. Inputs: rb = radius of base rt = radius of top of conical frustum h = height of the conical frustrum Outputs: F12 = view factor from interior wall to base A1 = area of the interior wall F21 = viewfactor from base to interior wall A2 = area of the base Reference: A Catalog of Radiation Heat Transfer Configuration Factors, third edition, John R. Howell, The University of Texas at Austin

37 37 / 43 Using the Function cone2base.m Thermal Network Model r b = m, r t = m and h = m >> [F12, A1, F21, A2]=cone2base(0.1397,0.0889,0.0508) F12 = A1 = F21 = A2 =

38 38 / 43 View Factor Matrix, [F] Thermal Network Model A (m 2 ) heater target shield env 0.1 A good rule of thumb when setting the area for radiation to the surroundings, is to make it about the same order of magnitude as the enclosure area. heater target shield env heater target shield env

39 39 / 43 Using the Function scriptf.m Thermal Network Model >>help scriptf [sf] = scriptf(emiss, F) Description: Evaluate the "script-f" or transfer factors for an enclosure radiation problem, given the geometric view factors and surface emissivities. Input: emiss(:) = vector of surface emissivities - length(emiss) = number of enclosure surfaces F(:,:) = geometric view factor matrix - can be a full or sparse matrix Output: sf(:,:) = script-f matrix - if F is sparse, then sf will be sparse

40 40 / 43 Using the Function scriptf.m Thermal Network Model >>emiss = [0.8, 0.1, 0.28, 1.0]; >>F = [ 0, , , ; , 0, , ; , , 0, ; , , , ] >>[sf] = scriptf(emiss, F);

41 41 / 43 Exchange Factor Matrix, [F ] Thermal Network Model ɛ heater 0.8 target 0.1 shield 0.28 env 1.0 heater target shield env heater target shield env

42 42 / 43 Thermal Network Solution Results env 25 (C) s-e (W) t-e (W) water 100 (C) shield (C) h-e (W) t-w (W) t-s (W) target (C) h-s (W) h-t (W) heater 525 (C)

43 43 / 43 Conclusion Provided view factor formulations required for the experiment Derived the relationship between the exchange factor, F, and the Oppenheim network Developed the thermal network model of the radiation heat transfer experiment Demonstrated simulation results for preliminary experimental data Questions?

44 44 / 43 References I [BLID11] T.L. Bergman, A.S. Lavine, F.P. Incropera, and D.P. DeWitt. Introduction to Heat Transfer. John Wiley & Sons, New York, sixth edition, [Gil02] [Hot54] David G. Gilmore, editor. Spacecraft Thermal Control Handbook, Volume I: Fundamental Technologies. The Aerospace Press, El Segundo, California, second edition, H. C. Hottel. Radiant-heat transmission. In Heat Transmission [McA54], chapter 4, pages

45 45 / 43 References II [HS67] H. C. Hottel and A. F. Sarofim. Radiative Transfer. McGraw-Hill, New York, [LL12] J. H. Lienhard, IV and J. H. Lienhard, V. A Heat Transfer Textbook. Phlogiston Press, Cambridge, Massachusetts, fourth edition, Available at: [McA54] W. H. McAdams. Heat Transmission. McGraw-Hill, New York, third edition, 1954.

46 46 / 43 References III [TD70] Y. S. Touloukian and D. P. DeWitt. Thermal Radiative Properties: Metallic Elements and Alloys, volume 7 of Thermophysical Properties of Matter. IFI/Plenum, New York, Engineering Library, Reference, Floor 1, QC171.P83.

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