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HIROPEAM Report Summary

Project ID: 323453
Funded under: FP7-JTI
Country: France

Final Report Summary - HIROPEAM (High rotational heat pipe experimental analysis and modelisation for turbomachine purpose)

Executive Summary:
Th whole description of the developed works, analysis and results are provided in the final report in attached document. Hereafter we give main elements of this report.

In rotating machines, heat generated inside the rotor is in general very difficult to evacuate because of bad thermal connection to the stator and the difficulty to apply an efficient cooling fluid flow. In addition, both machines densification and the increasing of their rotation speed make that point highly critical.
An interesting solution is to provide optimized exchange zones at the rotor end. As a consequence, the need arise to transport heat flux from the central into the end regions, with a minimum temperature gradient.
Taking into consideration the compactness and simplicity constraints, heat pipes present a highly effective solution. Their operating principle, which consists on the exploitation of the latent heat associated with the liquid-vapour phase change of a working fluid, allows heat transport within very small temperature gradients. Moreover, the fluid flow is produced by a pressure gradient at the vapour phase and a capillary pumping in the liquid phase, which eliminates any mechanical system and is thus very reliable.
The use of heat pipes in rotating systems was proposed since the 70’s by Gray et al. [1] for rotation speeds up to 10 000 rpm. Rotating heat pipes (for which axis of rotation coincides with the heat pipe axis) and revolving heat pipes (for which axis of rotation is different from the heat pipe axis) use the centrifugal force in order to ensure the return of liquid from condensing to evaporation area. The evaporation area must be farther from the axis of rotation than the condensation area, therefore.

Due to strong experimental constraints few experimental results, concerning rotating heat pipes, are found in the literature. In this context, three teams in particular have developed active research dealing with rotating heat pipes performance.

In early 70's [2] and until late 80’s [3], PJ Marto et al. investigated different possibilities of implementing heat pipe in rotating machines. Both initial modelling and experimental results have shown the large potential of this approach. However, these studies are limited to low speeds and provide no answers as high rotation speeds range is concerned.

In the late 90’s, Ponnapan et al. [4-5], made performance measurements for higher rotation speeds ranging from 10 000 to 30 000 rpm. Even here, however, due to experimental constraints, most measurements were limited to moderate rotation speeds. The difficulty of measuring temperature and applied powers makes it difficult to use those results in a straightforward manner. However, these experiments do confirm the correct operation of rotating heat pipes even at high rotation speeds.
Finally, more recent work of Song et al. |6-7] , focused on the modelling of exchanges in the smooth rotating heat pipes. They have shown the importance of taking into account the Rayleigh-Bénard convection cells in order to evaluate accurately the heat exchange at the evaporator.
Literature on high speed heat pipes is very limited, therefore, and provides only very general information about the performance of these heat pipes.
[1] V. H. GRAY - The rotating heat pipe- a wickless hollow shaft for transferring high heat fluxes. Journal of Heat Transfer, 69-HT-19, 1969
[2] P.J. Marto, T. J. Daley and L.J. Ballback - An analytical and experimental investigation of rotating non-capillary heat pipe. Annual report NPS-59 MX 70061A Monterey California, US naval Postgraduate School AD 709907, 1970
[3] P.J. Marto and A.S. Wanniarachi - Influence of internal axial fins on condensation heat transfer in co-axial rotating heat pipes Heat transfer and fluid flow in rotating machinery, pp 235-244, 1987
[4] R. Ponnapan, J.E. Leland, J.E. Beam – Experimental results of high-speed on-axis rotarting heat pipe Proceedings of the 9th Internationnal Heat pipe Symposium, Albuquerque, 1995
[5] R. Ponnapan, Q. He, J.E. Leland – Test results of water and methanol high-speed rotating heazt pipes Journal of thermophysics and het transfezr, 12(3) 1998

[6] F. Song, D. Ewing, C.Y. Ching – Fluid flow and heat and transfer model for high-speed rotating heat pipes International Journal of Heat and Mass Transfer 46(23) 2003
[7] F. Song, D. Ewing, C.Y. Ching – heat transfer in the evaporator section of moderate-speed rotating heat pipes International Journal of Heat and Mass Transfer 51(7-8) 2008

Project Context and Objectives:
Recent modelling developments of high speed rotating heat pipes conducted within the laboratory require experimental validation that the laboratory wishes to pursue. Moreover, improvements in the existing models should allow a significant extension of their validity range by taking into account the vapour and other modes of heat transfer at the evaporator.

Because of acceleration gradient in the vapour phase and the possible presence of noncondensable gases, many phenomena can occur, interfere, impede or prevent the operation of the heat pipe. In the evaporator, due to the temperature gradient with respect to the centrifugal force, transfer modes other than the liquid film conduction may appear. Thus for very high acceleration, the Rayleigh-Bénard convection can occur and boiling phenomena are likely to appear in the occurrence of important wall overheating. Based on experimental results, these aspects should be incorporated into the model.

The first scientific objective is to validate experimentally the assumptions made in the operating model of rotating heat pipes.

The results of the first model developed in the laboratory show that the condensation zone has very high thermal resistance in all circumstances. So it seems fundamental to intend to optimize the internal geometry of the heat pipe. Many options exist and will be tested numerically, analysed financially and finally the most promising of them will be tested experimentally.

In order to validate models with good confidence, the experimental setup will allow to change the operating temperature over an as wide as possible range, depending on the power transferred, however.

It appears that most of the work will focus on conducting a test bench in highly constraining conditions of rotating heat pipe at speeds up to 60 000 rpm. The conditions for balancing are all drastic and do not allow the use of embedded sensors nor to ensure good thermal insulation of the heat pipe from the room. A special effort must be made, therefore, on a nodal modelling of the entire test bench in order to take into account all exchanges around the heat pipe.

Finally, the behaviour of heat pipes in transient regime may be investigated to validate the onset and warm-up especially when starting at negative temperatures with water as coolant.
An external assistant can be provided for the integration of the candidate heat pipe on a demonstration machine.

Project Results:

a) Timeline & main milestones
An experimental bench was developed for testing the two-phase system under the conditions which are the most similar to the real operating conditions.
This bench constitutes the demonstrator of the project.
The investigational testing of the smooth heat pipe showed that this system is effective in difficult operating conditions. In addition, a parametric study was carried out to determine the influence of several parameters: the nature of the fluid, water is slightly more effective than ethanol; the filling ratio; over the range studied, the filling rate has little impact on heat pipe performances; the rotational speed, it does not disturb the general operation of the heat pipe; cooling system of condenser, air blast over the condenser improves thermal performances especially at low speed of rotation.
The overall thermal performances of the first smooth rotating heat pipe are very interesting. The nodal model has enabled to upoint out that the condenser area chiefly is limiting the heat transfer. Indeed, thermal conduction in the liquid film thickness in the condenser area decreased efficiency of two-phase systems. To reduce this phenomenon, the internal geometry of the rotating heat pipe is grooved, grooves acting as fins, and the thermal conductivity of the stainless steel wall is 25 times greater than that of water.
The influence of several parameters has been studied on grooved rotating heat pipe: nature of fluid, rotation speed, power applied and filling mass. The nature of fluid (water, ethanol, FC72) influences weakly the performances of grooved rotating heat pipe. Rotation speed does not influence the system except for rotation speeds lower than 10,000 rpm; below this rotation speed, a part of evaporator area was dried out. The filling mass is the parameter most influencing. Indeed, when it is higher than 100%, temperature oscillations appear. These oscillations seem to be outcome of the non axiality of rotating heat pipe; the period of these oscillations correspond to the rotation speed. The best performances were observed for a filling mass near 50%.
The thermal performances of both rotating heat pipes are very interesting. Furthermore, the second rotating heat pipe which grooved is even more efficiency.

From a numerical point of view, a solver has been developed with the following new features: a new phase change model and the multi-domain. The new phase-change model is a kinetic theory-based model.
The fully three-dimensional smooth rotating heat pipe was simulated using all the necessary recipe ingredients but the simulation time was too prohibitive (in a 40-processor parallel run, simulating 1 physical second took about 7 computation days) to tackle correctly both the complete unsteady regime, after the rotating heat pipe load is applied, and the pseudo-steady state.
An axi-symmetric configuration of the same heat pipe was then considered in order to reduce the computation time. Here, the simulation time was reduced to about 24 computation hours in order to simulate typically 5 physical seconds. This allowed us to unfold the entire unsteady regime, but there the heat pipe was not able to reach a steady state at all, which prevents valuable results to be obtained and useful conclusions to be drawn as the effect of the speed of rotation, wall thickness and shape, wettability, filling ratio and other fluid and solid properties on the overall performance of the rotating heat pipe.
Besides the development of numerical models and tools, this important work allowed us identifying the main modelling issues linked with the complex phenomena involved in rotating heat pipes. Some of these were addressed (interface conductivity, spurious current, curvature interface accuracy) but others, more complicated (mass conservativeness, flow stability issues), have not been definitely solved during the HIROPEAM project time period. These issues have to be dealt with necessarily but out of the present framework.

In addition, an elaborated liquid film model was developed and simulated. In contrast with the classical and usual lubrication theory, the full Navier-Stokes equations were used to model the liquid flow in order to take into consideration the buoyancy effect under centrifugal effects. The Arbitrary Lagrangian-Eulerian (ALE) method was used in order to describe the moving liquid-vapour interface where capillary and thermocapillary forces are considered. In this case, two main phenomena may compete: the stabilizing centrifugal effect, which tends to maintain the liquid phase farther away from the axis of rotation than the vapour phase and therefore flattens the liquid-vapour interface, on the one hand, and the destabilizing buoyancy and thermocapillary effects, which tend to induce Rayleigh-Bénard/Marangoni instabilities, on the other hand. It was found that mainly Rayleigh-Bénard destabilizing effect significantly affects the system for moderate rotational speeds (~4000 rpm), where convective rolls appear at the evaporator and both: i) disturb the drainage of the liquid from the condenser zone to the evaporator zone and, ii) enhance liquid mixing. An analysis of dependence of the RHP conductance on the rotational speed was also performed. In was shown that the larger the rotational speed, the larger the RHP conductance, other things being equal, with two distinguishable operating regimes; a moderate rotational speed regime (<4000 rpm), which is characterized by very small change in RHP conductance and a high rotational speed regime (> 4000 rpm), for which significant change in RHP conductance, as the rotational speed increases, is noticed. Note that no of these flow patterns can be captured by the classical lubrication models.
Connection between the liquid film flow pattern and the RHP performance was strongly suggested.
In fact, for the low rotational regime, two convective rolls (one at the evaporator/adiabatic transition region and the other at adiabatic/condenser transition region) get more intense for increasing rotational speed. Nevertheless, the localness of these rolls makes them not an effective contributing factor on the RHP performance. For higher rotational regime, substantial enhancement in rotating heat pipe performance was shown to be influenced also by the occurrence of instabilities at the evaporator area, where several convective rolls develop all along and so mixing is significantly increased. The Critical Rayleigh number associated with this instability was found to be close to 400, which corresponds (for an applied heat power set to 25 W and a film mean thickness about 1 mm) to a rotational speed about 4,000 rpm. So reducing the film thickness by a factor 2 (and so reducing the mass of the working fluid by approximately the same factor) makes instability happen for 16,000 rpm. One must then keep in mind that, as far as RHP is concerned and in addition to drying out increasing risk, it is not straightforward that reducing the film thickness will systematically reduce the film resistance, since convective effects may strongly interfere with pure conductive ones.

Intensification of efforts on the experimental aspect (expertise, more complementary tests) has led today to geometry of rotating heat pipe very effective and producing noticeable performance enhancing. In the near future, the numerical tools to be developed will guide a further optimisation of the rotating heat pipe characteristics and so will help an even more enhancing of its performances.
The numerical developments done so far in this project were disseminated though one national
Conference, one international conference, and one article submitted to peer-reviewed journal, up to now under review.
As a perspective, the numerical tool that was developed in the HIROPEAM framework has to be enhanced in the near future in order to:
1. Consider in a more reliable and physical way the critical issue of mass conservation;
2. Optimize robustly the parallel processing of the solver in order to handle truncated (and may be full) three-dimensional grooved heat pipe;
3. Reduce even more the spurious current and increase accuracy in evaluating the liquid-vapour interface curvature and normal direction;
4. Include a more realistic wettability, i.e. contact angle, physics (may be in a multiscale manner).

Potential Impact:
Development of a zero-energy consumption heat dissipation device for rotating machines

The device installed in the experimental setup has proved its appropriateness and is therefore provided as a demonstrator.

1 F. Menu, Y. Bertin, A. Benselama, and C. Romestant. Vers un outil de simulation tridimensionnelle des transferts de chaleur et de masse dans un caloduc tournant. In SFT Editor, Société Française de Thermique, 2015, pages 1-8, 2015.
2 A. M. Benselama, C. Romestant, Y. Bertin, and V. Ayel. Investigation of thermogravity/ thermocapillary effects in rotating heat pipes: prediction of instabilities at the liquid. ln. IHPC India, Oct. 2013.
3 A. M. Benselama, C. Romestant and Y. Bertin, A consistent-conservative scheme for diffuse-interface approach of heat and mass transfer in two-phase flows. Submitted to International Journal for Numerical Methods in Engineering

List of Websites:
No website

Professor Yves Bertin ISAE-ENSMA Institute Pprime UPR 3346
1 avenue Clément Ader BP 40109 86961 Futuroscope Chasseneuil Cedex FRANCE
(33) 0549498123 ;

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