Announcement by Travis Potter

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers

About me ASHRAE Student Chapter President 2017-2018 Co-Op as Freese and Nichols, Inc. December 2016 – August 2017 Experience in Energy Modeling, Energy Audits, and HVAC Design Contact: Travis.S.Potter@UTexas.edu About me

Local society of mechanical engineers specializing in Architectural Mechanical, Electrical and Plumbing. Local Mentors Scholarships ~$10,000 available (went unallocated last year) Networking Opportunities Career Opportunities WHAT, WHY?

WHen, WHere? 7th Floor Room 7.208 Thursday, October 5th 6pm -7pm Nick Prisco of Freese and Nichols, Inc will be presenting on chilled water VAV systems. WHen, WHere?

Energy efficiency will continue to be more important in the coming years Public health is influenced by our built environments Job Security: Typical Structural Design Lifespan: 50-75 years Typical HVAC Design Lifespan: 20-25 years Why mep?

Questions? 7th Floor Room 7.208 Thursday, October 5th 6pm -7pm Nick Prisco of Freese and Nichols, Inc will be presenting on chilled water VAV systems. Contact: Travis.S.Potter@UTexas.edu Questions?

Lecture Objectives: Answer questions related to HW 2 Write energy balance equations Show matlab example Start analyzing unsteady state Conductin

Homework assignment 1b Top view Teast_o Teast_i Tinter_surf Tair_in Styrofoam Teast_o Tinter_surf Teast_i 2.5 m Tair_in Surface to surface radiation 8 m 8 m IDIR Idif Tsouth_i South East Tsouth_o IDIR Tair_out Idif Glass Surface to surface radiation

MatLab Example Example code is posted on the class website MatLab at: http://www.ece.utexas.edu/it/student-licensing

Conductive heat transfer k - conductivity of material Steady-state Unsteady-state Boundary conditions Dirichlet Tsurface = Tknown Neumann TS1 TS2 L h Tair

Unsteady-state heat transfer (Explicit – Implicit methods) Example: To - known and changes in time Tw - unknown Ti - unknown Ai=Ao=6 m2 (mcp)i=648 J/K (mcp)w=9720 J/K Initial conditions: To = Tw = Ti = 20oC Boundary conditions: hi=ho=1.5 W/m2 Tw Ti To Ao=Ai Conservation of energy: Time [h] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 To 20 30 35 32 10 15 Time step Dt=0.1 hour = 360 s

Implicit methods - example After rearranging: 2 Equations with 2 unknowns!  =0 To Tw Ti  =36 system of equation Tw Ti  =72 system of equation Tw Ti

Explicit methods - example  =360 sec  =0 To Tw Ti  =360 To Tw Ti  =720 To Tw Ti Time There is NO system of equations! UNSTABILE

Problems with stability !!! Often requires very small time steps Explicit method Problems with stability !!! Often requires very small time steps

Explicit methods - example  =0 To Tw Ti  =36 To Tw Ti  =72 To Tw Ti Stable solution obtained by time step reduction 10 times smaller time step Time  =36 sec

Explicit methods information progressing during the calculation Tw Ti To

Unsteady-state conduction - Wall q Dx Nodes for numerical calculation

Discretization of a non-homogeneous wall structure Section considered in the following discussion Discretization in space Discretization in time