1. Announcement by Travis Potter

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

3. 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:
About me

4. 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?

5. 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?

6. 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: years
Typical HVAC Design Lifespan: years
Why mep?

7. 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:
Questions?

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

9. 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

10. MatLab Example Example code is posted on the class website MatLab at:

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

12. 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

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

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

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

16. Explicit methods - example
 = 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

17. Explicit methods information progressing during the calculationTw Ti To

18. Unsteady-state conduction - Wallq Dx
Nodes for numerical
calculation

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