1 Absorber Heat Transfer and Other Issues A Comparison between MICE and the Forced Flow Absorber System Michael A. Green Lawrence Berkeley National Laboratory Berkeley CA 94720, USA MUCOOL Workshop Meeting Fermilab, Batavia IL, USA 22 February 2003

2 A Summary of MICE Absorber Issues The Heat transfer on the helium side is forced convection in the absorber case tube. The flow of the helium is set by the flow from the refrigerator. Heat transfer on the hydrogen side is by free convection. Buoyancy determines the mass flow of the sub-cooled hydrogen. The hydrogen mass flow goes up as the heat load (Q B + Q H ) to the 0.5 power. The change in the bulk hydrogen temperature goes as the heat load to the 0.5 power. Freezing the liquid hydrogen in the absorber is not allowed when there is no heat load into the absorber, so the helium enters the absorber body at 14 K.

3 A Summary of Forced Flow Absorber Issues The Heat transfer in the forced flow absorber heat exchanger between the helium gas and the sub-cooled hydrogen in the absorber flow circuit is marginally OK. The position of the heat exchanger with respect to the absorber and the hydrogen pump is of concern. The condensation of liquid hydrogen into the absorber circuit can be a key operational issue. One can circulate the liquid hydrogen through the absorber by natural convection. One should be able to remove up to 1000 W of heat from the absorber using natural convection.

4 A Simplified Schematic of the MICE Absorber Heat Loads Q C = 20 W Q R = 10 W Q B +Q H = 70 W Q T = 100 W Window T > 55 K

5 Thermal Modeling the MICE Absorber using an Electrical Network Analogy Q C = 20 W Q R < 10 W Q T = 100 W Q B + Q H > 70 W T I > 55 K when Q R = 0 T f < 20 K T R = 300 K

6 Desired

7 A Comparison of the MUCOOL Forced Flow Absorber with the MICE Free Convection Absorber

8 Possible MICE Absorber Heat Exchangers

9 Counter Flow Heat Exchangers versus Parallel Flow and Mixed Heat Exchangers In a parallel flow heat exchanger, the coldest temperature of the warm stream is always higher than the warmest temperature of the cold stream. This restriction does not apply for a counter flow heat exchanger. For a given heat exchanger U factor and heat exchanger area, a counter flow heat exchanger will nearly always have the lowest log mean temperature difference. A mixed flow heat exchanger has all of the disadvantages of a parallel flow heat exchanger plus the heat exchange and hydrogen flow in the absorber is unbalanced. Heat from the outlet stream is shorted to the colder inlet stream. In situations where a change of phase occurs on one side of the heat exchanger, any type of heat exchanger works.

10 Parallel Flow and Counter Flow Heat Exchangers

11 An Estimate of the Pumped Hydrogen Forced Flow Heat Exchanger U Factor The U factor for the MICE absorber Heat Exchanger is much lower.

12 A MICE Absorber Heat Exchanger

13 MICE Peak Bulk LH 2 Temperature Vs 14 K Helium Mass Flow and Heat Load into the Absorber Inlet He T = 14 K

14 MICE Peak Bulk Helium Temperature Versus the Heat Load into the Absorber Helium Inlet T = 4.3 K Note: The two-phase helium flow is at least 3.5 g/s.

15 The pumped Hydrogen Forced Flow Absorber Configuration Studied The configuration shown was given at last week’s telephone meeting. This week’s configuration is not the same. The new configuration will have improved performance.

16 Forced Flow Peak Bulk Hydrogen Temperature Vs Helium and Hydrogen Mass Flow for Q =225 W

17 Forced Flow Peak Bulk Hydrogen Temperature Vs Helium and Hydrogen Mass Flow for Q = 375 W

18 Problems with the Pump Loop The heat exchanger area is too small. Increasing heat exchanger area will reduce the log mean temperature difference and improve efficiency. The pump flows against buoyancy forces. The heat exchanger will flood as hydrogen is condensed into the pump loop. As result, hydrogen condensation will slow to a snails pace.

19 A Better Pump Loop Solution The heat exchanger area is increased a factor of three. As a result, the system is more efficient. The pump and the heat exchanger are oriented to use buoyancy forces to help hydrogen flow. The top of the heat exchanger is above the liquid level. The heat exchanger is an efficient hydrogen condenser.

20 Can a Free Convection Loop be used? Circulation of the hydrogen using free convection should be seriously considered. Preliminary calculations suggest that up to 1000 kW can be removed from the absorber using a free convection loop. The hydrogen flow through the absorber is proportional to the square root of the heat removed. The bulk hydrogen temperature rise is proportional to the square root of the heat removed. The heat exchanger must be vertical with the hydrogen flowing in the downward direction. The helium will flow in the upward direction. The top of the heat exchanger should be above the hydrogen liquid level. It is not clear if a free convection hydrogen flow loop will fit in the lab G solenoid.

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22 Some Concluding Comments The MICE absorber appears to be OK for heat loads to the hydrogen of up to 100 W. (30 W is transferred to the helium gas directly.) The MICE absorber appears to work with liquid helium in the absorber with total heat loads up to about 45 W. (30 W goes to the two-phase helium directly.) The new MUCOOL forced flow experiment will work as designed. The flow experiment works because the mass flow in the both streams of the loop is larger than the optimum. Increasing the MUCOOL pump loop heat exchanger area will improve the pump loop heat transfer efficiency at lower hydrogen mass flows. Correct orientation of the pump and heat exchanger should improve the loop performance. A free convection hydrogen loop appears to be feasible. A free convection loop may not fit into the lab G solenoid.