MAN Bus A37: HVAC Analysis & Benchmark with THESEUS-FE (based on thermal simulation using the finite element method) Dr. S. Paulke, P+Z F.

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1 (based on thermal simulation using the finite element method) Dr. S. Paulke, P+Z F. Artmeier, MAN Truck & Bus AG Dr. K.-E. Yildirim, MAN Truck & Bus AG Dr. V. Bader, VW-Konzernforschung Prof. Dr. A. Gubner, Hochschule für angewandte Wissenschaften München Fakultät für Maschinenbau, Fahrzeugtechnik, Thermodynamik Date:

2 Project description Length Height Width Our goal was to demonstrate the capabilities of THESEUS-FE for simulating a bus AC system. In a first step we validated the Finite Element results of THESEUS-FE by comparing them with the results obtaining using an MAN in-house tool. The MAN tool, developed and tested by Mr. Felix Artmeier as part of his diploma thesis in 212, considers energy balances for the bus cabin. In a second step we demonstrated the advantage of employing THESEUS-FE by creating an extended simulation model also capable of simulating passenger thermal comfort. Finally we validated the THESEUS-FE prediction of AC power using a Mollier diagram. Conclusion: we were able to show that THESEUS-FE is a useful simulation tool in designing a bus AC system. Environmental conditions for any place on earth are easily integrated into the model. Project partners: P+Z Engineering GmbH Volkswagen Research MAN Slide 2

3 Environmental Boundary Conditions THESEUS-FE permits all kinds of thermal boundary conditions such as conduction, convection, and radiation. The overall intent of the simulation was to derive the net power of the HVAC system needed to enforce a constant value of 2 C cabin temperature (T cabin ) typical climate conditions for a summer day in Germany. See slides 12/13. For the extended THESEUS-FE model we added seats and additional heat sources such as passenger metabolism and engine heating. Mean bus velocity: v=2 km/h heat conduction through walls & windows external convection engine heat T sky = -15 C HVAC: cool air outlet T out internal: thermal radiation via view factors metabolic heat from passengers T cabin =2 C T ground = 4 C internal convection solar absorbed solar transmitted external: thermal radiation heat exchange with the environment (sky, ground, clouds, walls of buildings,...) T wall = 3 C Slide 3

4 Environmental Boundary Conditions Realistic thermal boundary conditions for a summer day from 6: until 19: in Germany: Clear sky: no clouds Driving direction: southwards Moving sun position T amb : Ambient air temperature [ C] Temp_Verlauf 6: 1: 14: 18: azi : Solar azimuth PHI_AZI angle [deg] I dir : Direct solar intensity I_DR [W/m 2 ] : 1: 14: 18: 6: 1: 14: 18: alti : Solar altitude angle [deg] PHI_ALTI : 1: 14: 18: I dif : Diffuse solar intensity [W/m 2 ] I_DF : 1: 14: 18: Slide 4

5 Finite Element Model THESEUS-FE graphical users interface: Lion s City Bus (A37) Mean edge length in model: 6cm Shell elements in model: Solid elements in model: 51 8 Temperature degrees of freedom in model: x z y Radiation view factors: (for internal radiation) Passenger seats: Time step size:.5 h Engine CPU time for 13 h real time simulation: 117 s Slide 5

6 Finite Element Model Finite element types used in the MAN bus model for THESEUS-FE n Shell element multi-layered conduction, including air-layers n window 1..5 transmittance t 3 t 2 t 1 absorb. reflec. outside c p1, 1, 1 c p2, 2, 2 c p3, 3, 3 inside. 3 6 phi 9 TRANS REFLEC ABSOB Solid element tetrahedral, 3D conduction Slide 6

7 Validation vs. Simplified Energy Balance Model The Simplified Energy Balance Model (SEBM) was developed by Felix Artmeier during his diploma thesis at MAN in 212. To make the THESEUS-FE model comparable to the SEBM we excluded metabolic and engine heating. Local results (element heat fluxes & temperatures) can not be validated because the SEBM only yields global heat fluxes and mean values for temperatures. Outside surface temperature, mean value: [ C] T sf Outside convective heat flow: [W] Outside long-wave radiation heat flow: [W] Q conv + Q rad + SEBM Slide 7

8 Validation vs. Simplified Energy Balance Model The validations demonstrate a good fit between the two simulation tools! Post-processing with THESEUS-FE enables the visualization of all kinds of local results directly on the 3D model in the THESEUS-FE graphical user interface (GUI). See the figure on the right. Pre-processing with the THESEUS-FE GUI enables fast and direct model changes with a few mouse clicks. THESEUS-FE results at day time 8: Outside solar absorbed heat flow: [W] Solar transmitted heat flow: [W] Q sun,ab + Q sun,tr + Slide 8

9 Validation vs. Simplified Energy Balance Model The table beneath explains the major differences between THESEUS-FE and SEBM... THESEUS-FE Model changes fast and intuitive Transient simulations Internal wall temperatures available Solar transmission: fast ray tracing Detailed bus model based on FEM Internal cabin air humidity available Solar transmission and absorption depend on the angle of incidence (see slide 6) Ray tracing for solar transmission & reflection Inside the cabin: Thermal radiation via view factors Multi-layered finite elem. (incl. air-layers) 1D or 3D heat conduction models Thermal comfort prediction available SEBM Parametric model changes in Excel Only quasi-static simulations... not available Solar transmission: no ray tracing Simplified models based on global heat balances Internal cabin air humidity not available Solar transmission and absorption do not depend on the angle of incidence... not available Inside the cabin: Thermal radiation can not be considered Multi-layered model without air-layers 1D heat conduction models...not available Slide 9

10 Advanced THESEUS-FE Model In a second step we introduced an extended THESEUS-FE model and added additional heat sources: Engine heating 35 C fixed wall temperature 33 passengers each with 87W metabolism applied on each seat area (.34m 2 ) 87/.34 = 256 W/m 2 The thermal comfort index PMV* will be derived for the drivers position. A sf * PMV: predicted mean vote introduced by Fanger (197) Slide 1

11 Advanced THESEUS-FE Model The PMV strongly depends on a theoretical radiation (wall) temperature T w that will derived from the long-wave (Q rad,ab ) and short-wave radiation (Q sun,ab ) absorbed by the reference surface A sf on the drivers seat: Q rad,ab Q sun,ab 4 T 4 w Tsf C, Tsf in Kelvin A sf sf w At 1: the heat load at the drivers seat reaches it s maximum... THESEUS-FE temperatures at day time 1: THESEUS-FE thermal comfort prediction hot 3 warm 2 PMV T w slightly warm neutral 1 6: 1: 14: 18: 4 2 Slide 11

12 Advanced THESEUS-FE Model The overall goal of the simulation was to derive the net power of the HVAC system needed to enforce a constant value of 2 C cabin temperature 4 2 6: 7: 8: 9: 1: 11: 12: 13: 14: 15: 16: 17: 18: 19: Coarse THESEUS-FE Simplified THESEUS-FE (Model model 6) Simplified Energy Balance Model (SEBM) Simplified Energy Balance (Model 6) Fine THESEUS-FE Advanced THESEUS-FE (Model model 8) [W] -18 H heat 16kW Fine THESEUS-FE model with additional Advanced THESEUS-FE (Model 8 with add. heat heat sources) sources (engine, passengers) The remaining differences between THESEUS-FE and SEBM results stem from the fact that the SEBM cannot perform ray tracing for solar radation. Slide 12

13 HVAC Validation using Mollier Diagram air temperature [ C] rel. humidity : HVAC at 15: day time T amb 1 HVAC Mollier Diagram abs. humidity: x = m steam /m air Fresh air ventilation Flow rate (12m 3 /h ): 3 m kg kg s m s m T cabin T out 1 1 T amb = T 1 = 22.5 C amb = 1 = 67 %, x 1 = 11 g/kg h 1 = 51 kj/kg T out = T 2 =.7 C out = 2 = 1 %, x 2 = 4 g/kg h 2 = 1kJ/kg T cabine = T 3 = 2 C cabine = 3 = 27.5 %, x 3 = 4 g/kg h 3 = 3kJ/kg 2 HVAC: Net power of cooling: H heat kj kg h m 41.4 kg s 16kW Slide 13