Heat Pipe Characteristics

From Thermal-FluidsPedia

 Related Topics Catalog

Historical Development of Heat Pipes Operation Principles of Heat Pipes Types of Heat Pipes Working Fluids and Temperature Ranges of Heat Pipes Capillary Wick Designs and Structures in Heat Pipes Heat Transfer Limitations of Heat Pipes Heat pipe Start Up Heat Pipe Characteristics Heat Pipe Analysis and Simulation Heat Pipe Applications

Contents 1 High Effective Thermal Conductivity 2 Thermal Transformer 3 Power Flattening 4 Temperature Control 5 Freedom of Design 6 Maintenance-Free 7 References

High Effective Thermal Conductivity

The heat pipe is a device of very high thermal conductance ^[1][2]. For example, a temperature difference of 900°C is needed to transfer 1 KW heat across a 30-mm-diameter 1-m-long copper rod. A heat pipe of the same size can transfer the same amount of heat with a temperature difference of less than 10°C. This indicates that the heat pipe can have a thermal conductivity 90 times higher than that of a copper bar of the same size. Numerous heat pipe designs have been developed for various applications with varied heat transport capabilities.

The heat transfer characteristics of a heat pipe can be measured using a conventional configuration as shown in Fig. 1(a). Figure 1(b) shows the heat transfer rate versus the mean temperature difference between the evaporator and condenser sections of commercially available copper-water heat pipes with a screen mesh wick and different diameters. Most of the tests were made in a vertical position (gravity-assisted wicked heat pipe), where the driving force is due to both capillary pumping and gravity. A typical test for horizontal operation, as well as a comparison with a copper rod of 24.5 mm OD, are also shown in Fig. 1(b).

The thermal response of a conventional commercial screen mesh copper-water heat pipe with 6.35 mm OD and an inner diameter of 5.85 mm in comparison with a copper pipe of the same dimension is shown in Fig. 2. There are commercial companies all over the world (with more concentration in the Far East) who manufacture heat pipes of different shapes and sizes for various operating temperature ranges. A typical commercial off-the-shelf performance characteristic for several copper water heat pipes (Fujikura, Ltd) is presented in Fig. 3.

Three of the important design parameters that are used to compare heat pipes and other two-phase thermal devices are the thermal resistance, the axial heat transport capacity (W-m) and the maximum radial heat flux at evaporator (W/cm²). Table 5 presents typical experimental operating values reported in literature for a number of heat pipes tested under various operating conditions. The operating temperature, wick design, wall material and dimensions of the heat pipes tested are listed in this table. Also, the axial heat transport rate (W), axial heat transport capability (W-m) and the radial heat flux (W/cm²) are given. The radial heat flux or axial heat transport values reported may not necessarily be the limiting values.

Thermal Transformer

The heat pipe can be effectively used as a thermal transformer to link energy sources and sinks having different heat fluxes. Energy can be added at a high heat flux rate to the heat pipe over a small surface area and removed over a larger surface area at a lower heat flux, or vice versa. Thermal flux transformation ratios as large as 15 to 1 can be achieved using heat pipes.

Power Flattening

A heat pipe that provides power flattening can be easily designed. A constant output heat flux at the condenser can be maintained for variations in the input heat flux in the evaporator section.

Temperature Control

By actively determining the conditions at the condenser (sink), a heat pipe can be designed to keep a nearly constant temperature at the evaporator even though the rate of heat input to evaporator varies. This is the basis for the variable conductance heat pipe which is discussed by Faghri ^[2]. The gas-loaded heat pipe, in which a noncondensible gas and a gas reservoir are constructed with a conventional heat pipe, is the simplest means to actively determine the conditions at the condenser.

Freedom of Design

Since the basic mechanisms of a heat pipe are the capillary pumping of a wick and the evaporation and condensation of a working fluid, heat pipe design is only restricted by the need to provide for these mechanisms. With this in mind, heat pipes can be designed so that they fit various shapes and configurations. These geometries range from simple cylindrical or flat heat pipes, to curved plate heat pipes for solar collectors and leading edge heat pipes.

Maintenance-Free

Since heat pipes are a closed systems and do not need any external electrical or mechanical drives, they can operate over long periods of time without maintenance.

Table 5 Some typical experimental heat pipe results
Working Fluid	Operating Temperature	Wick Design	Wall Material	Dimensions	Axial Heat Transport (W)	Axial Heat Transport Capability (W-m)	Radial Heat Flux (W/cm2)	Reference
Methane	-140°C	Circumferential mesh wick and arterial wick	Stainless steel	Lt = 0.4 m Le = 3 cm Lc = 11.5 cm Di = 7 mm	12	3.93	1.82	Rosler et al. [3]
Ammonia	25°C	Monogroove	Aluminum	Lt = 5.5 m Le = 0.91 m Lc = 0.91 m Dv = 13.4 mm Dl = 6.32 mm slot width = 0.381 mm	700	3200	7.26	Alario et al. (1982)[4]
Water	50°C	Axial grooves	Copper	Outer pipe same as above; Inner pipe: Do = 29.7 mm Di = 25.4 mm	1300	940	Inner pipe: 2.32 Outer pipe: 1.47	Faghri and Thomas (1989) (annular heat pipe)[5]
Water	50, 70, 80°C	3 wraps of a copper wire screen mesh with a wire diameter of 0.11 mm and 3937 strands per meter	Copper	Do = 19.05 mm Di = 15.75 mm Lt = 355.6 mm Le = 101.6 mm Lc = 152.4 mm	20-180	4.6-41	0.33-2.96	Kempers et al. (2008)[6]
Water	60-95°C	Micro capillary grooves (trapezoidal and rectangular)	Copper	Lt = 120.65 mm W = 13.41 mm Le = 15.6 mm Lc = 34.4 mm La = 70 mm	130 (horizontal) 170 (vertical)	15.6 20.4	90 (horizontal) 150 (vertical)	Hopkins et al. (1999) [7]
Water	75°C	Circumferential copper screen (50 mesh)	Copper	L = 1.0 m Le = (4)@6.4 cm Lc = 30 cm Dv = 25.4 mm Di = 22 mm	392	283	1.92	Faghri and Buchko (1991) (multiple evaporators) [8]
Water	100°C	Axial grooves	Copper with rectangular cross section	Lt = 120 mm H = 2 mm W = 7 mm Le = 20 mm Lc = 20 mm	70	8.4	35	Plesch et al. (1991) [9]
Water	100°C	Copper wick, 150 mesh	Copper	Do = 19.1 mm Di = 17.3 mm Lt = 610 mm Le = 393 mm Lc = 170 mm	570	187	2.4	El-Genk and Huang (1993) [10]
Water		Axial rectangular grooves	Copper	Lt = 82 mm	40		20	Cao et al. (1997) [11]
Water	100 °C	Axial rectangular grooves	Copper	Lt = 120 mm W = 7 mm H = 3 mm	50 (horizontal) 70 (vertical)		25 (horizontal) 35 (vertical)	Gao et al. (2000) [12]
Water	160°C	Double-wall artery	Copper	Lt = 1.2 m Le = 0.2 m Lc = 0.2 m Dv = 13.4 mm Do = 22.2 mm	900	900	6.45	Faghri et al. (1984) [13]
Therminol VP-1	300-400°C	Thermosyphon	316-stainless steel	Do = 15.8 mm Di = 6 mm Lt = 209 mm Le = 45 mm Lc = 100 mm	35-70	4.8-9.6	1.57-3.13	Jouhara and Robinson (2009) [14]
Sulfur-Iodine 5% wt	350°C	Thermosyphon	Mild steel	Do = 25 mm Di = 20 mm Lt = 1000 mm Le = 400 mm Lc = 550 mm	390	204.8	1.24	Trovole and Raine [15]
Working Fluid	Operating Temperature	Wick Design	Wall Material	Dimensions	Axial Heat Transport (W)	Axial Heat Transport Capability (W-m)	Radial Heat Flux (W/cm2)	Reference
Sodium	430-790°C	Circumferential stainless steel screen (100 mesh)	Stainless steel	Lt = 1.0 m Le = (4)@5.3 cm Lc = 29.2 cm Do = 26.7 mm Di = 21.5 mm	1309	979	6.64	Faghri et al. (1991a) (multiple evaporators) [16]
Sodium	650°C	Nickel sintered powder metal wick and an artery	Stainless steel	Do = 21.3 mm Di = - Lt = 460 mm Le = 100 mm Lc = 160 mm Lc,inactive = 100 mm	1400	322	21	Yamawaki et al. (1998) [17]
Sodium	800°C	Double-wall artery	Stainless steel	Lt = 2.0 m Le = 25 cm Lc = 91 cm Do = 2.22 cm	758	1076	4.35	Ponnappan (1989) [18]
Mercury	630°C	Thermosyphon	316L stainless steel	Do = 25.4 mm Di = 21 mm Lt = 1000 mm Le = 200 mm Lc = 640 mm	1922	1115	12	Vieira da Cunha and Mantelli (2009) [19]
NaK (with Argon as NCG)	700°C	304 stainless wick	304L stainless steel	Do = 19.1 mm Di = - Lt = 600 mm Le = 114 mm Lc,1 = 73 mm Lc,2 = 66 mm Lc,inactive = 50 mm	250	57	3.65	Anderson and Tarau (2008) [20]
Lithium	1227°C (1500 K)	Free-floating Mo-41wt.%Re 400-mesh screen wick (0.41 mm thick) A 0.37 mm annular gap separates the wick from the wall	molybdenum	Do = 19.1 mm Di = 16.06 mm Lt = 1800 mm Le = 300 mm Lc = 1470 mm	4000	3660	22.2	Tournier and El-Genk (2003) [21]

References

1. ↑ Faghri, A., 2012, "Review and Advances in Heat Pipe Science and Technology," Journal of Heat Transfer, 134(12), 123001. http://dx.doi.org/10.1115/1.4007407
2. ↑ ^{2.0 2.1} Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.
3. ↑ Rosler, S., Groll, M., Supper, W., and Konev, S., 1987, "Analysis and Experimental Investigation of a Cryogenic Methane Heat Pipe," Proceedings of the 6th International Heat Pipe Conference, Grenoble, France, 219-222.
4. ↑ Alario, J., Haslett, R., and Kosson, R., 1982, "The Monogroove High Performance Heat Pipe," Progress in Astronautics and Aeronautics, 83, 305-324.
5. ↑ Faghri, A., and Thomas, S., 1989, "Performance Characteristics of a Concentric Annular Heat Pipe: Part I-Experimental Prediction and Analysis of the Capillary Limit," Journal of Heat Transfer, 111(1-4), 844-850. http://dx.doi.org/10.1115/1.3250795
6. ↑ Kempers, R., Robinson, A. J., Ewing, D., and Ching, C. Y., 2008, "Characterization of Evaporator and Condenser Thermal Resistances of a Screen Mesh Wicked Heat Pipe," International Journal of Heat and Mass Transfer, 51(25-26), 6039-6046. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2008.04.001
7. ↑ Hopkins, R., Faghri, A., and Khrustalev, D., 1999, "Flat Miniature Heat Pipes with Micro Capillary Grooves," Journal of Heat Transfer, 121(1), 102-109. http://dx.doi.org/10.1115/1.2825922
8. ↑ Faghri, A., and Buchko, M., 1991, "Experimental and Numerical Analysis of Low-Temperature Heat Pipes with Multiple Heat Sources," Journal of Heat Transfer, 113(3), 728-734. http://dx.doi.org/10.1115/1.2910624
9. ↑ Plesch, D., Bier, W., Seidel, D., and Schubert, K., 1991, "Miniature Heat Pipes for Heat Removal from Microelectronic Circuits," Proceedings of the ASME Winter Annual Meeting, Atlanta, GA.
10. ↑ El-Genk, M. S., and Huang, L., 1993, "An Experimental Investigation of the Transient Response of a Water Heat Pipe," International Journal of Heat and Mass Transfer, 36(15), 3823-3830. http://dx.doi.org/10.1016/0017-9310(93)90062-B
11. ↑ Cao, Y., 1997, "A Feasibility Study of Turbine Disk Cooling by Employing Radially Rotating Heat Pipes," Final Report for Summer Faculty Research Program, Air Force Research Lab, Turbine Engine Division.
12. ↑ Gao, M., Cao, Y., Beam, J. E., and Donovan, B., 2000, "Structural Optimization of Axially Grooved Flat Miniature Heat Pipes," Journal of Enhanced Heat Transfer, 7(6), 361-369.
13. ↑ Faghri, A., Stewart, R. J., and Rainey, C. L., 1984, "Axial Variation of Local Heat Flux Along the Condenser Section of a Double Wall Artery High Capacity Heat Pipe," Proceedings of the 5th International Heat Pipe Conference, Tsukuba, Japan, 13-17.
14. ↑ Jouhara, H., and Robinson, A. J., 2009, "An Experimental Study of Small-Diameter Wickless Heat Pipes Operating in the Temperature Range 200c to 450c," Heat Transfer Engineering, 30(13), 1041-1048. http://dx.doi.org/10.1080/01457630902921113
15. ↑ Trovole, H. P., and Raine, J. K., 1995, "The Design and Heat Pipe Tests for a Line Focus Solar Stirling Domestic Generation System," Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 209(1), 27-36.
16. ↑ Faghri, A., Buchko, M., and Cao, Y., 1991a, "A Study of High-Temperature Heat Pipes with Multiple Heat Sources and Sinks: Part I - Experimental Methodology and Frozen Startup Profiles," Journal of Heat Transfer, 113(4), 1003-1009. http://dx.doi.org/10.1115/1.2911193
17. ↑ Yamawaki, S., Yoshida, T., Taki, M., and Mimura, F., 1998, "Fundamental Heat Transfer Experiments of Heat Pipes for Turbine Cooling," Journal of Engineering for Gas Turbines and Power, 120(3), 580-587. http://dx.doi.org/10.1115/1.2818186
18. ↑ Ponnappan, R., 1989, "Studies on the Startup Transients and Performance of a Gas Loaded Sodium Heat Pipe," Technical Report, WRDC-TR-89-2046.
19. ↑ Vieira da Cunha, A. F., and Mantelli, M. H., 2009, "Analytical and Experimental Analysis of a High Temperature Mercury Thermosyphon," Journal of Heat Transfer, 131(9), 1-7. http://dx.doi.org/10.1115/1.3089551
20. ↑ Anderson, W. G., and Tarau, C., 2008, "Variable conductance heat pipes for radioisotope stirling systems," Proceedings of STAIF 2008, 969, 679-688. http://dx.doi.org/10.1063/1.2845031
21. ↑ Tournier, J. M., and El-Genk, M. S., 2003, "Startup of a Horizontal Lithium-Molybdenum Heat Pipe from a Frozen State," International Journal of Heat and Mass Transfer, 46(4), 671-685. http://dx.doi.org/10.1016/S0017-9310(02)00324-1