Development of heat pipes with potassium as woking fluid: Performance limitations and test rig development
The incentive to reduce energy consumption in the industry is big, especially
in high temperature systems. Heat pipes are of great interest for this purpose
due to their favorable thermomechanical properties. This master thesis is a
part of the ongoing study of Ph.D. candidate Geir Hansen, who is currently
developing a rectangular heat pipe with potassium as working fluid at NTNU.
The rectangular heat pipe is intended to be implemented in the walls of
electrolysis cells as a part of a heat recovery system.
The present work reports results of theoretical calculations of two important
heat transfer limitations, the incipience of boiling and the capillary
limitation for two types of nickel foam wicks. Results of experimental tests
carried out on the cooling circuit for the proposed rectangular heat pipe are
also reported. The foam porosity, permeability and effective pore radius for
wick 1 is 0.797, 31·10−12m2 and 62·10−6m, respectively, and for wick 2; 0.886,
205·10−12m2 and 126·10−6m.
A literature survey showed that porous coated surfaces improves the heat
transfer and requires less superheat for boiling to commence. Calculations
performed showed no danger of homogeneous nucleation in the proposed heat
pipe. Boiling inside the nickel foam wick(s) were found to only be of concern
for wick 2 at high heat fluxes and a operating temperature of 600C.
Calculations of the capillary limit showed that wick 2 is the best choice
for sustaining high heat fluxes. Increasing the wick length to 20cm made
wick 2 not suitable for usage, and wick 1 was the best choice for increased
wick length. Combination of the two wick types showed to be very effective
and significantly (factor of almost 4) improved the performance. An uneven
heat flux distribution where a lower heat flux is at the bottom region of the
evaporator is found to lower the performance, while a higher heat flux at the
bottom region increases the performance.
Early tests revealed that the PID controller was marginally stable, so the
controller was tuned and stable operating conditions were achieved. Experiments
showed that in order to get an accurate heat balance for the test rig,
knowledge about the exact position of the thermocouples is needed.
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2012. , 140 p.
ntnudaim:8178, MTPROD produktutvikling og produksjon, Energi-, prosess- og strømningsteknikk
IdentifiersURN: urn:nbn:no:ntnu:diva-18600Local ID: ntnudaim:8178OAI: oai:DiVA.org:ntnu-18600DiVA: diva2:566114
Næss, Erling, ProfessorHansen, Geir