Thermal Performance Comparison Between Water

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Eliel, W.2, Riehl. Roger, R.3 ... large amounts of heat by capillary forces over considerable distances with no need of additional power input to the .... Tests were performed at power levels of 25W, 50W, 75W, 100W and 125W during a period of.
Thermal Performance Comparison Between Water-Copper and Water-Stainless Steel Heat Pipes Silva. Debora de O.1, Marcelino. Eliel, W.2, Riehl. Roger, R.3 National Institute for Space Research - INPE, São José dos Campos, SP, BRAZIL 12227-010

Heat pipes are high capacity heat transport systems built essentially by three components: the external enclosed structure (container), the working fluid and wick structure. Heat pipes have become highly reliable systems and since 1970, heat pipe technology has been widely applied in several areas, such as heat exchangers, spacecraft thermal control and cooling systems for electronic components. This technology has found increasing application in improving the thermal performance of heat exchangers in many industrial environments. The use of heat pipes in heat exchangers allows the development of more compact and efficient equipments, when compared to traditional heat exchangers. For some applications, such as heat recovery in industrial process, the use of the heat pipes on heat exchangers presents to be rather interesting due to its direct influence on increasing the efficiency, allowing a more compact design for those. In this scenario, this paper presents an experimental investigation of heat pipes designed and manufactured in both stainless steel and copper using water as working fluid for mid-level temperature range. Results show that the copper heat pipes demonstrated higher thermal conductanceswhen using screen mesh number 100, while stainless steel heat pipes showed higher thermal conductance values by using screen mesh number 200. Even though water-copper heat pipes usually presents a better thermal performance when compared to the water-stainless steel heat pipes, still there are a wide range of applications for stainless steel heat pipes.

Nomenclature HP1 HP2 HP3 HP4 SS Gevap_cond T_evap T_cond Q Pcap σ θ r

= = = = = = = = = = = = =

heat pipe 1 heat pipe 2 heat pipe 3 heat pipe 4 stainless steel Evaporator-condenser thermal conductance (W/oC) evaporator temperature (oC) condenser temperature (oC) applied heat in evaporator section (W) capillary pressure surface tension (N/m) contact angle (o) pore radius (m)

I. Introduction

T

HE use of heat pipes was initially made by aerospace industry due to its high capacity of heat transport through capillary forces in microgravitational fields, but due to increasing of energy costs industries in a general manner started looking for energy saving alternatives and energy management solutions for better cost-benefit relations1. Therefore, the application of heat pipes means one of those efficient solutions which industry is always looking for. Cooper-Water heat pipes operating at mid-level temperature range used in this investigation represents one of the 1

PhD Student, Space Mechanics and Control Division, Av dos Astronautas, 1758. PhD Student, Space Mechanics and Control Division, Av dos Astronautas, 1758. 3 Senior Research Engineer and Faculty, Space Mechanics and Control Division, Av dos Astronautas, 1758. 1 American Institute of Aeronautics and Astronautics 2

alternatives for several industry applications, also leaving grounds for the application of staintless-steel-water heat pipes. Since the first heat pipes conception made by Gaugler (1944)1, the investigation and application of this type of heat transport device has become wider within industry. Heat pipes are thermal devices, which efficiently transports large amounts of heat by capillary forces over considerable distances with no need of additional power input to the system1.Capillary pressures in heat pipes tends to be higher with smaller pore sizes of the wick structure; on the other hand, larger pores are preferred for the liquid movement within wick structure due to a higher permeability. This means that wick structure mesh and pore dimensions are not quite simple to be designed and indeed demands specific investigations depending on the application. In a general manner, there are three most relevant wick properties to be check out during the concept design phase: minimum capillary radius, permeability and effective thermal conductivity1 so that, each chosen mesh of wick structure will bring different challenges and results in terms of their capillary limit, boiling limit, entrainment limit, vapor pressure limit, etc. These wick properties in a heat pipe design are aimed to allow the return of liquid from condenser to the evaporator as well as to develop the capillary pumping pressure by providing surface pores at liquid-vapor interface and finally to allow the existence of a heat flow path between liquid-vapor interface and inner wall of container.2 The material selection and compatibility between wick and container as well as the working fluid are major factors in the design of heat pipes. Both cooper heat pipes and stainless steel heat pipes present to have a good material compatibility level with water2. Thus, this paper brings a comparison between water-cooper and water-stainless steel wick structures.Even though water-copper heat pipes as known to present better thermal performances when compared to the waterstainless-steel heat pipes, one advantage of stainless steel over cooper is a smaller weight parameter. The weight parameter is evaluated in terms of density divided by the ultimate tensile stress of material, so for obtaining a lighter material then a smaller weight parameter value is desirable2. The experimental tests performed and presented in this paper considered the wrapped mesh type for wick structure, which usually is the most commonly used in practice, also two mesh numbers were considered. The mesh number is determined by measuring the number of meshes contained in one linear inch perpendicular to the wire. Also, it is known that mesh size is specified in terms of mesh number3. The different mesh numbers can bring different meanings in terms of both liquid-flow resistance and capillary pumping. The liquid-flow resistance is inversely proportional to the square of mesh size in wick structure and capillary pumping pressure is inversely proportional to mesh size2.In this study the thermal conductance will be one of the parameters under discussions and comparisons between both water-copper and water-stainless steel heat pipes.

II. Heat Pipe technology and operation principle The operation of a heat pipe can be understood by assuming a cylindrical geometry, as shown in Fig.1. The components of a heat pipe are a sealed container, a wick structure, and a calculated amount of working fluid which is in saturated condition. As shown by Fig.1, the heat pipe is divided into three parts: evaporator section, adiabatic section, and condenser section. The heat applied to the evaporator section by an external source is conducted through the pipe wall and wick structure, where it vaporizes the working fluid. The resulting vapor pressure drives the vapor through the adiabatic section to the condenser, where the vapor condenses, releasing its latent heat of vaporization to the provided heat sink. The condensed liquid then returns to the evaporation section by capillary forces, closing the loop.3 For heat pipe applications, the heat source and sink conditions are usually specified previously so that the heat pipe design can be tested and properly evaluated. Therefore the design feasibility and thermal effectiveness of a given heat pipe for a specific application can be evaluated, by determining the operating conditions for a given source and sink condition.4 There are several parameters which establishes some limits and constraints for both steady-state and transient heat pipe operation conditions. Some physical phenomena that may influence on the heat transport into heat pipe operation are due to capillary limit, sonic limit, entrainment limit, boiling limit, frozen startup, continuum vapor, vaporpressure and condenser effects. Any of those heat transfer limitations can affect the heat pipe operation depending on the size and shape of the pipe, working fluid, wick structure and wick type, and operating temperature. Additional and detailed explanations regarding factors that affects the heat pipe operation such as proper material compatibility, weight, temperature characteristics, and fabrication, and material costscan be found on many other publications, such as Peterson1,Reay2and Chi4.

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Figure 1. Components and principle of operation of a conventional heat pipe4

III. Experimental apparatus In order to investigate the potential application of heat pipes for industrial using, anexperimental benchwas built,which was able to perform heat pipe tests at any inclination between -90 and 90o, with a good precision level by using a digital leveling instrument, so that the heat pipe tests could be properly performed within these inclination values. Thebenchwas made up with four heat pipes, being two heat pipes built ofstainless steel316-L and two heat pipes of copper (Fig.2). The geometric characteristics for the four tested heat pipes are described in the Table 1. The experimental test bench consists of four heat pipes, a DC power controller (Agilent N5749A) and a National Instrument SCXI data acquisition system controlled by LabVIEW. Temperatures were measured at various positions at the tubes wall, using six Omega T-type thermocouples (accuracy of ±0.3oC at 100oC) per tube, in two locations of the evaporation, adiabatic and condenser sections. Another thermocouple was issued for measuring the ambient temperature (Fig. 2b). Heat was applied to the heat pipes using an electric resistance wrapped around the evaporation section and connected to the digital DC power supply. The condenser was open to the ambient air, exchanging heat by natural convection. All tests were performed at controlled room conditions, with temperatures of 22oC ± 2oC. Thus, oscillations on the ambient temperature were expected due to the air conditioning on/off operation.

Figure 2. a) Experimental bench with heat pipes, data acquisition system and power controller; b)Positionsofthermocoupleson theheatpipesanddata acquisition system. Regarding the heat pipe materials, the copper and stainless steel 316L were chosendue to their wide application within industryand also due to the acceptable compatibility with water, mainly for copper. Each heat pipe was designed using wire mesh as the wick structure, which was responsible for returningthe condensed liquid to the evaporator by capillary forces. Usually the selected wick structure depends on the heat pipe design and it has important features which usually affects the heat transport performance, basically due to the fact that the wick structure purpose is to generate enough capillary pressure to transport the working fluid from the condenser back to 3 American Institute of Aeronautics and Astronautics

the evaporator. According to the Young-Laplace relation from Eq. (1), the smaller the pore radius the higher capillary pressure, on the other hand, the higher pore radius the less liquid movement restriction. ܲ௖௔௣ =

ଶఙ௖௢௦ఏ ௥

.

(1)

At the meantime, the smaller radius usually are responsible to increase the pressure drop of the fluid across the wick structure, which can be a serious limitation for the heat pipe operation due to the decrease of permeability as well. Thus, pore radius must be carefully investigated for each application so that a reasonable commitment can be determined as far as the overall heat pipe operation is a concern. Finally, a comparison between water-copper and water-stainless steel heat pipes was performed based on the use of the same screen mesh numbers for both copperwater and stainless steel heat pipes. The HP1 is a stainless steel 316L heat pipe and the HP3 is a copper heat pipe, however both carrying a screen mesh number of 100. The HP2 is a stainless steel heat pipe and the HP4 is a copper heat pipe, and both carry a mesh screen number of 200. The operating temperature range and operating power levels were the same for all four tested heat pipes. Table 1. Geometric Characteristics of the Heat Pipes Heat Pipes Characteristics

HP1 / HP 3

Evaporator/Adiabatic/Condenser/Total length (m)

HP2 / HP 4 0.25 / 0.9 / 0.35 / 1.5

Working Fluid

Water 316L – SS / Copper

Tube material Outer / Inner diameter (m)

316L – SS / Copper

0.0191 / 0.0135

Screen wick material

316L - SS / Copper

316L - SS / Copper

Screen mesh number

100

200

Number of wick layers Wick Porosity %

/

3 2

Permeability (m )

Mean pore radius (m)

-9

-9

0.68 / 2x10

0.64 / 6x10

-4

-5

1.27x10

6.35x10

Operating temperature range (°C)

22 - 160

Operating Power (W)

25 - 125

IV. Results and discussions After the performed tests, it was possible to verify the temperatures versus time along the heat pipes' length (evaporator, adiabatic and condenser) were checked and analyzed in order to obtain the temperature profiles related to the applied power. Tests were performed at power levels of 25W, 50W, 75W, 100W and 125W during a period of 5,000 seconds per each applied power level. Figure 3a demonstrates the temperature profiles for stainless steel heat pipes HP1 and HP2 while Fig. 3b demonstrates the temperature profiles for copper heat pipes HP3 and HP4. All heat pipes were tested at a chosen bench test inclination of zero degree, so that they all could be verified at the same operation condition. The oscillations observed during the start-up process can be considered normal when water is used as working fluid, also the effects regarding the meniscus formation and equilibrium between the evaporation and condensation processes must be generated in the interface. Since that equilibrium is established, the heat pipe reaches a more stableoperation with small or even without oscillations of the applied heat to the evaporator5. In terms of temperature profile for stainless steel heat pipe HP1 which uses a screen mesh number of 100 (Fig.3a), the comparative results demonstrated a temperature difference between evaporator and condenser specifically for applied power levels between 25W and 75W, while it was observed that this temperature difference was decreased for power levels of 100W and 125W. The heat pipe HP2, which uses a screen mesh number of 200, had a similar behavior when compared to HP1 in terms of temperature profile, however it was observed that HP2 heat pipe showed a higher temperature difference between evaporator and condenser for power level of 25W, which means that for power levels above 25W (50W to 125W) the temperature difference between evaporator and condenser meaningfully decreased and became close to one another. 4 American Institute of Aeronautics and Astronautics

(a) (b) Figure 3. Temperature profiles for stainless steel heat pipes HP1 / HP2 (Fig. 3a) and copper heat pipes HP3 / HP4 (Fig. 3b) This behavior discussed above for HP1 and HP2 can drive to at least two parameters that affects heat pipe operation: liquid-flow resistance and capillary pumping. As per Faghri3, the liquid-flow resistance is inversely proportional to the square of mesh size in wick structure and capillary pumping pressure is inversely proportional to mesh size.Thus, by comparing HP1 that uses screen mesh number of 100 to HP2 that uses screen mesh number of 200, the liquid-flow resistance will be higher for HP1 than liquid-flow resistance in HP2, so that the HP1 has a smaller speed of liquid return from the condenser back to evaporator. This makes the temperature differences to be higher for power levels from 25W to 75W, while for power levels of 100W and 125W, the vapor pressure become higher due to increase of heat, which makes the temperature differences between evaporator and condenser become close to each other. In the same way, for HP2 the liquid-flow resistance is lower than HP1 due to its higher screen mesh number, which tends to the fact to decrease the temperature difference in a uniform way along almost the whole temperature profile for the applied power to the evaporator(from 50W up to 125W). At the meantime when analizing Fig.3b, it can be observed that the temperature profile for copper heat pipes HP3 and HP4, which uses a screen mesh number of 100 and 200 respectively, demonstrated an opposite behavior than stainless steel heat pipes HP1 and HP2. This happens mainly due to the fact that the initial applied power levels of 25W and 50W demonstrated a very small temperature difference between evaporator and condenser, while for the remaining tests (from 75W up to 125W), the temperature difference between evaporator and condenser tends to became higher. In a general manner, the adiabatic region was kept almost the same along the temperature profile between HP1 and HP2 steainless steel heat pipes (Fig. 3a) and also between HP3 and HP4 copper heat pipes (Fig. 3b). Figure 4a demonstrates the thermal conductance results for stainless steel heat pipe HP1 versus copper heat pipe HP3 in which both HP1 and HP3 use the very same screen mesh number of 100, while the Fig. 4b demonstrates the thermal conductance results for stainless steel heat pipe HP2 versus copper heat pipe HP4. Both HP2 and HP4 use the very same screen mesh number of 200, so thermal conductance results are driven by heat pipe material. Due to the fact the experimental results may raise possible variations which came from heat pipe manufacturing standpoint and it affects heat pipe operation, and also knowing the heat pipes used for this study were manually manufactured, then an adjustment factor was taken into consideration for thermal conductance results so that we could get a more accurate analysis in a way to minimize these possible variations. The adjustment factor is defined as the ratio of the heat applied (Q) to the device by the temperature difference between the evaporator (T_evap) and condenser (T_cond), with deviation of ± 1,22% for HP1, ± 2,06% for HP2 [5], ±1,9% for HP3 and 1,01% for HP4 being given by the following relationship: .

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(2)

(a) (b) Figure 4. Thermal conductance results for screen mesh numbers of 100 in HP1 and HP3 (Fig.4a) and screen mesh numbers of 200 in HP2 and HP4 (Fig. 4b) The uncertainty on the calculated results from Eq. (2) are less than 8%. When evaluating the results from Fig. 4a, it was possible to verify that thermal conductance results for HP3 (copper heat pipe) are higher along all the testing for the applied power levels than HP1 (stainless steel heat pipe) thermal conductance. Figure 4b demonstrated that copper heat pipe HP4 has higher thermal conductance when compared to HP2 stainless steel heat pipeat 25W and 50W power levels. The range obtained for the tests were in horizontal orientation between 1.0 and 7.7 W/°C for HP1, 1.45 and 14.0 W/°C for the HP2, 3.4 and 17.4 W/°C for HP3 and 4 and 8.4 W/°C, adjusting one variation of ±51%, ±25%, ±17,4% and respectively. The use the adjustment to refine the model of heat pipes, aims its use in applications indicated, with important information for integrated simulation of heat pipes with heat exchangers and other aerospace devices. With the results for thermal conductance, the HP3 presented higher thermal conductance compared to HP1 which was somehow expected, based upon the temperature profile graphs demonstrated in the Fig 3a, due to the use of number 100 mesh. On the other hand, the HP2 compared to HP4 presented higher conductance values from 75W and above due to lower end to end temperature drop between the evaporator and the condenser in this region already mentioned in the temperature profile graphs. Results shows better thermal performance for copper heat pipes when mesh number is 100, while SS heat pipes demonstrated better thermal performance with a higher mesh number of 200. The highest thermal conductivity observed during the current tests was 20 W/°C obtained at a power of 125 W for HP3.

V. Conclusion Experimental results for heat pipes HP1 and HP2 (stainless steel) and HP3 and HP4 (copper) average operating temperature using water as a working fluid, with mesh numbers 100 and 200. With the experimental results,the thermal adjustment was calculated to obtain the thermal conductance of the heat pipe to ain order to provide a direct comparison between the stainless steel and copper heat pipes.The following conclusions can be derived from this investigation: 1.

2.

3.

4. 5.

The HP1 demonstrates smaller thermal conductance when compared to HP3 for power level of 100 Wdue to lower end to end temperature drop between the evaporator and the condenser; however, with the adjustment factor, HP1 reached thermal conductances values close to HP3 thermal conductance; Although it was observed different thermal conductance values between HP2 and HP4, the adjustment factor for the HP2 and HP4 kept thermal conductance values close to each other after the startup oscillations shown in the HP2 for power level of 50W; Although there is a difference in the experimental thermal conductance between HP2 and HP4 pipes, due to a larger temperature difference between the condensers and the evaporators of the two heat pipes, the adjusted results show values close for both devices tested; Stainless steel heat pipes demonstrated a smaller temperature drop for power levels above 75W; Copper heat pipes demonstrated smaller temperature drop for power levels of 25W and 50W; 6 American Institute of Aeronautics and Astronautics

6. 7.

Regarding the thermal conductances, the copper heat pipes demonstrated higher values by using screen mesh number of 100; Stainless steel heat pipes demonstrated higher thermal conductance values by using screen mesh number of 200.

These initial results provide some early insights for the operation conditions of the tested heat pipes and also some initial highlights on a few heat pipe characteristcs which might be relavant mainly for comparison and possible choices of stainless steel heat pipes and copper heat pipes. However, further investigations and developments are undergoing in order to provide reliable operation for the proposed heat pipe characteristcs, in such a way that they can meet industry level requirements.

References 1 Peterson, G. P., An Introduction to Heat Pipes Modeling, Testing, and Applications, John Wiley & Sons, INC, New York, 1994, Chaps. 2, 3, 7, 8. 2 Reay, D. A., Kew, P. A., Heat Pipes-theory, design and applications, 5a Ed., Oxford, UK: Elsevier’s Science & Technology, 2006. 3 Faghri, A., “Heat Pipe Science and Technology,” Taylor and Francis, Washington, DC, 1995. 4 Chi, S.W., Heat Pipe Theory and Practice, Hemisphere Publishing Corporation, Washington, 1976. 5 Silva, D. de O., and Riehl R. R., “Development of Heat Pipes Operating at Mid-Level Temperature Range Applied for Industry,” AIAA 2014-3769, 2014.

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