Global thermal model for an off-set strip fin
intermediate heat exchanger (IHX)
Modeled with:
Compact Heat Exchanger Explicit Thermal and Hydraulics Code
CHEETAH Code
Developed by Graduate Student:
Eugenio Urquiza Fernández (ME)
&
Research Advisors (1/2006-finish):
Prof. Per Peterson (NE)
&
Prof. Ralph Greif (ME)
Doctoral Research (start Jan. 2006): Advanced High Temp. Reactor (AHTR)
Objective: Develop a detailed transient and steady state thermo-mechanical model for a compact heat exchanger applied to a vastly improved next generation nuclear reactor
Nuclear power technology is poised to safely supply the world’s electrical energy demands on the scale currently generated using combustion of fossil fuels while avoiding the greenhouse emissions of fossil fuel dependant methods. The advanced high temperature reactor is a new generation reactor using liquid salts to transfer heat from a nuclear reaction to a closed loop multiple reheat helium brayton cycle for power generation. Unlike other reactor designs the AHTR’s liquid salt loops isolate the core form the power generation and avoid phase change and thus high pressure near the reactor.
Figure 1: The Advanced High Temperature Reactor power/hydrogen plant
Image: Prof. Per Peterson - Nuclear Engineering Department at UC Berkeley
This multiple reheat helium brayton cycle can provide upwards of 50% thermodynamic efficiency and would be significantly cheaper than comparable steam turbine cycles. The proposed system calls for heat to be delivered between 800°C and 1000°C. The elevated temperature requires the implementation of highly compact heat exchanger that retains its strength well above the prescribed temperatures. Currently, a melt-infiltrated carbon-carbon composite heat exchanger is proposed but its thermal response has yet to be modeled in detail.
The research project entails designing and modeling (thermally and mechanically) a compact heat exchanger that can effectively transfer heat from a closed liquid salt loop to a closed high pressure helium loop. The compact heat exchanger thus must be designed to safely deal with the high temperature salt, temperature transients, and the operating pressures across the heat exchanger.
The design of the compact heat exchanger will be optimized to both minimize pump work and maximize heat transfer. Heat transfer modeling in the compact heat exchanger will involve both the forced convection and natural convection scenarios that represent operating conditions and reactor ramp up and shut down. The natural convective properties of the liquid salts (Grashof & Prandt Number) have been shown to match those of certain light mineral oils in previous scaling experiments. With the fluid properties established full thermal models of the heat exchanger will be constructed.
Once nuclear reactor is shut down the core continues to generate some heat; thus cooling system is designed to reject heat without pump power through natural convective flows. The compact heat exchanger thermal models are thus essential to the total system thermal model as they model a significant mode of reactor heat rejection. This total system model will then predict the transient temperature behavior of the reactor during shut down. This spatial and time dependent solution will then guide design specifications for the advanced high temperature reactor.
Graphics – Proposed
Design Concept of Compact Heat Exchanger
The off set strip fin design for the liquid salt and high pressure helium can be seen in the two figures below by David Huang.
Figure 2: Liquid (molten) salt and helium plate geometries
Figure 3: Cut-away of the IHX plate assembly showing a representative unit cell of the off-set strip fin region of the IHX
The highly tortuous pathways resemble a highly ordered ‘porous medium’ and thus can be modeled as a Darcian flow by assigning calculating an effective permeabilities throughout the IHX geometry. The porous media assumption permits the researcher to model the flow and heat transfer in this heat exchanger with many of the same methods commonly used to model subsurface flows in enhanced oil recovery or groundwater hydrology.
Both
fluids are treated as incompressible. This is obviously valid for the
liquid salt but also resonable for the high pressure helium since the Mach
number <.3. The equation of continuity (conservation of mass) is
used in conjunction with Darcy's transport equation (momentum equation)
to solve for the fluid velocity distribution in the complex IHX
geometry where the effects of cross flow and temperature dependent
fluid properties (such as viscosity) can be taken into account. With
the velocity distributions solved, an energy balance is preformed on
each phase in thousands of finite volumes created by the CHEETAH Code and
thus the temperature distribution can be determined. Together the finite volumes represent the entire heat exchanger.
A simple one dimensional transient thermal model of the offset strip fin portion of the IHX is modeled below in Figure 4.
Recently, (October 2007) the CHEETAH Code was modified so that it can now model the entire IHX. This includes the complex geometry inlet and outlet manifolds as well as the pipe sections which had to be solved in cylindrical coordinates and plotted onto the rectangular (Cartesian) grid.While it's difficult to see in the figure below, the thermal effect of flow maldistribution or non uniformity in the inlet and outlet manifold can be seen as well as the effect of the separated flow channels in the off-set strip fin regions.
The transient is shown in the following frames (0 sec - 1st row, 20 sec - 2nd row, 40 sec - 3rd row, 67 sec - 4th row, 100 sec - 5th row, 150 sec - 6th row)
Figure 5: 150 second
thermal transient in solid layer of HX after liquid salt pump trip
(left column) and after high pressure helium pump trip (right column)
Future Work:
These
temperature distributions will allow us to predict the mechanical
stress in the IHX via methods previously developed at UC Berkeley. To
do this, the temperature distribution must be ported to a FEA
code. The CHEETAH Code and subsequent mechanical analysis will provide
essential tools to optimize the effectiveness and mechanical
performance of the heat exchanger. Improved thermal performance can
have a significant effect on the entire plant's efficiency and thus
reducing the waste produced per unit of energy output. The HX will
also be made more cost effective by making them less resource intensive
as well as more robust and thus safer.
This
is an abstract for the upcoming Joint International Workshop on
"Nuclear technology and society- Needs for Next Generation" will be
held January 6 to 8, 2008, on the UC Berkeley campus:
Intermediate Heat Exchanger Dynamic Thermal Response Model
Eugenio
Urquiza-Fernández
Per
F. Peterson
Ralph Greif
U.C. Berkeley
Oct. 31, 2007
Abstract
This paper
presents UCB progress in developing a comprehensive thermal and fluid dynamics
model for the Next Generation Nuclear Plant (NGNP) intermediate heat exchanger
(IHX) and other compact heat exchangers. For
improved efficiency electricity generation and for nuclear hydrogen
applications, an IHX is required to transfer heat from high temperature and
high-pressure primary helium coolant to a power plant or hydrogen production
process.An intermediate heat transfer
loop is used for the purpose. Under
these conditions, plate-type heat exchangers with small flow channels, such as
the well known Heatric designs, are a major candidates because they can achieve
high power densities with small amounts of material, and can be fabricated
using a diffusion bonding process so that the entire heat exchanger has the
strength of the base material. However, these types of heat exchangers can be
susceptible to very large stresses during thermal transients, for example when
the flow of one fluid is interrupted abruptly. UCB has proposed a capillary shell and tube IHX configuration that could
have lower susceptibility to thermal shock. For all IHX options accurate analysis of global and local thermal
stresses are critical to evaluating the heat exchanger reliability and safety.
In order to
estimate the stresses in compact heat exchangers a comprehensive thermal and
hydraulic model is needed. The model
developed here uses an effective porous media (EPM) approach because the evaluation
of the detailed global flow with computational fluid dynamics (CFD) as well as finite
element methods (FEM) for the mechanical analysis, at the resolution scale of
the flow channels involves prohibitive computational time. The EPM fluid dynamics and heat transfer
computational code developed at UCB is called the compact heat exchanger
thermal and hydraulics (CHEETAH) code. CHEETAH solves for the transient temperature-distribution
in the IHX. This temperature
distribution can then be imported into a commercial finite element analysis
(FEA) code for mechanical stress analysis using the EPM methods developed
earlier by UCB for global and local stress analysis [2]. These simulation tools will also allow the
designer to optimize the heat exchanger design, to minimize the pressure drop
while maximizing the IHX’s thermal effectiveness, as well as to optimize the
mechanical performance of the IHX particularly as it relates to creep
deformation and transient thermal stresses.
Past Work: Masters Research (completed Spring 2006):
Research Advisor (8/2003-12/2005):
Prof. Kent S. Udell (ME)
Masters Research (completed Spring 2006):
Thermally Enhanced Recovery of an Oil Phase in Porous Media [MS Project]
Thermally enhanced oil recovery applied to environmental cleanup and enhanced petroleum recovery particularly applicable in heavy oils and tar sands. This research investigates the effect of varying the gravity number of steam flow (ratio of bond number and capillary number) in 3 phase flow (Steam, Water, Oil) in porous media to optimize recovery factors.