Experimental study of channel roughness effect in diffusion bonded pulsating heat pipes

Journal Pre-proofs Experimental Study of Channel Roughness Effect in Diffusion Bonded Pulsating Heat Pipes L.A. Betancur, M.J.P. Flórez, M.H. Mantelli PII: DOI: Reference:

S1359-4311(19)35767-9 https://doi.org/10.1016/j.applthermaleng.2019.114734 ATE 114734

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Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

18 August 2019 14 October 2019 28 November 2019

Please cite this article as: L.A. Betancur, M.J.P. Flórez, M.H. Mantelli, Experimental Study of Channel Roughness Effect in Diffusion Bonded Pulsating Heat Pipes, Applied Thermal Engineering (2019), doi: https:// doi.org/10.1016/j.applthermaleng.2019.114734

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Experimental Study of Channel Roughness Effect in Diffusion Bonded Pulsating Heat Pipes Betancur, L.A, Flórez M. J. P., Mantelli. M. H. Department of Mechanical Engineering, Federal University of Santa Catarina, Florianopolis 88040-900, Brazil Abstract This study investigates the influence of surface finishing on thermal performance of copper flat pulsating heat pipes (PHP). Two copper flat PHPs with overall dimensions of 208x150x5 mm3 were fabricated using the diffusion bonding technique. In both devices, the internal surfaces were modified. In the first one, the channel inner surface was sanded with standard sandpaper Grit N1200. In the second PHP (called hybrid), the evaporator zone was sanded with standard sandpaper Grit N100 and the condenser section with Grit N1200. Both devices counted ten parallel channels with hydraulic diameter of 2.5mm. Distillated water was used as working fluid. The transient thermal performance and thermal resistance were analyzed, for filling ratios of 10, 15, 25, 37.5, 50 and 75 % and for tilt angles of 0° and 90° bottom heat mode. Start-up, thermal stability and hysteresis were experimentally tested. Three regimes were observed: conduction, unstable, stable. In general, the hybrid PHP showed lower thermal resistances, except for the filling ratio of 75% at vertical position. The hybrid device shows stop-over instabilities at power inputs between 140 to 180W, at vertical position and low filling ratios. Higher surface roughness increases the number of vapor nucleation sites, which, in turn, improves the phase change at evaporator zone and facilitates the start-up conditions at low filling ratios. However, increasing roughness also rises pressure drops, mainly at high filling ratios. Keywords: Roughness, Thermal resistance, Stop-over, Start-up, Diffusion bonding. 1. Introduction As the electronic technologies advance, so does the need to keep them at acceptable temperature ranges. In turn, this motivates the development of heat control solutions, able to dissipate high concentrated heat [1], [2]. These heat dissipation devices must be reliable, low weight, and, for aerospace applications, independent of gravity. Among the several heat dissipation technologies, heat pipes (including conventional, loop heat pipes and capillary pumps), have being largely used [3],[4]. From the heat pipe family, a relatively new, wickless device, denominated pulsating heat pipes (PHP) has received increasing attention from researchers [9]. The PHPs, first presented by Akachi [5], take advantage of the design flexibility and high heat transfer capacity, while maintaining low fabrication costs [6],[7],[8]. A PHP consists of a meandered small diameter tube, in several turns, closed in its extremities. It is internally evacuated and partially filled with working fluid. A PHP is composed by three regions: evaporator, condenser, and an adiabatic section, which may or may not be present. The heat delivered in the evaporator causes the working fluid in liquid phase to evaporate. Due to the small tube diameter, confined boiling is expected, with the formation of randomly distributed liquid slugs and vapor plugs. The working physical principles of PHP are based on complex and chaotic two-phase flow phenomena. The inner pressure of large vapor bubbles formed in the evaporator, pushes liquid slugs to the condenser direction, transporting sensible heat. When the bubbles reach the condenser, they lose heat and condensate, transferring latent heat [10]. Actually, PHPs tend to be an isochoric system, and thus, the sensible heat in the liquid slugs is the main heat transfer mechanism, representing more than 80% of the total transferred heat [11],[12]. For high heat loads, annular unidirectional flow may be formed and thus, the latent heat transfer mechanism plays a larger role [11], [13], [14]. The thermal performance of PHPs is lower than those of a pure latent heat transfer devices (e.g. thermosyphons) but, still is superior to a solid conductive material [3].

Different authors have reported that a considerable improvement in the performance of PHP is achieved if, by some artifice, the pressures inside the device are unbalanced and the liquid flow is forced in a preferential direction. This can be obtained by capillary media or by introducing check valves [15]–[17]. These procedures decrease the temperature oscillations and gravity dependency while increasing the overall thermal performance. However, these modifications increase a PHPs manufacturing complexity and consequently, fabrication costs. PHPs can also be designed to guarantee easy and early start up conditions which sustain phase change mechanisms at evaporator and condenser zones , thus maintaining the oscillation regime, avoiding temperature increments [3], [13], [14]. Usually in flat plate PHPs, two plates are welded against each other. Grooves are machined in one of the plates before the welding. This allows unlimited channel design and any channel surface modification [16]. As the literature reports, surface finishing strongly affects the working fluid boiling process in the evaporator, as vapor nucleation sites can be created. The liquid-solid wettability (contact angle and meniscus radius) can be modified [18], by either chemical or mechanical processes. Coatings or porous media layers can also be applied. Studies of the effect of supehydrophilic CuO coatings on the wettability of PHP evaporator channels have been performed in the literature, concluding that coating increases the wettability, improving the thermal performance of the device [19]. Another work [20] showed that a hybrid pattern, i.e., the use of CuO (superhydrophilic coating) at evaporator and superhydrophobic surface at condenser regions reduces the overall thermal resistance. However, the fabrication of flat PHP still represents a challenge. Different technologies have been used to create reliable monolithic pieces, such as the additive manufacture and power bed fusion [18]. If modified surfaces are to be applied, special care must be taken so that the fabrication process does not damage the already modified surface. In that regard, the diffusion bonding has been considered as a viable option [16], [21], [22]. As diffusion bonding involves high vacuum and high temperatures, PHPs with modified surfaces by CuO coatings cannot be manufactured through this process, as the finishing suffers reduction under these aforementioned conditions. As such, mechanical surface modification presents itself as a suitable method, compatible with diffusion bonding process. Actually, increased wall roughness increases nucleation sites and helps to achieve an early start-up conditions [18], [23]. However, the pressure drops observed in liquid flows through rough channels are high, due to solid-liquid friction[24]. The present work aims to compare the thermal performance of two diffusion bonded flat plate PHPs fabricated by diffusion bonding, in which the channel surfaces have different roughness. The first was sanded over its entire channel length with standard sandpaper Grit N1200 and the other was sanded with standard sandpaper Grit N100 at the evaporator zone and with Grit N1200 on the rest (adiabatic and condenser sections). 2. Physical model In this section, a physical model is proposed to explain the importance of the roughness on the PHP performance. In the evaporator zone, liquid slugs are separated by vapor plugs, and most of the tube internal area is recovered by a thin layer of liquid film, similar to the layers found in the flow boiling in microchannels [25][26]. The heat transfer at evaporator zone depends on the liquid distribution along the surface (both in liquid slug and film configuration). Figure 1, which was based on [26], shows a PHP in horizontal position. One can see that the receding and advancing angles are not the same, depending on the direction of liquid slug flow. Also this figure shows that the liquid slug and the vapor plug can be divided into six regions. Region I shows a local dry-out in the vapor plug, with no liquid layer over the surface. In this region, conduction between wall and vapor is the only heat transfer mechanism. The local dry-out depends highly of the capacity of the liquid to wet the surface, e.g. of the surface wettability parameter and of the amount of liquid inserted (filling ratio). Region II represents the equilibrium thin film region, where the film presents thickness 𝛿𝑜.

Region III represents the section in which the evaporating thin film is thicker, with thickness 𝛿. In Region IV, known as meniscus region, is where a single receding meniscus of radius 𝑅𝑟, is observed. In the liquid slug Region V, the liquid forms a cylindrical plug, with the same radius of the tube. Finally, in region VI, an advancing meniscus of radius 𝑅𝑎 is observed. At the end of region VI, another dry out region (I) is observed and the liquid slug and vapor plug pattern repeats itself. One of the most accepted surface characterization methods is the roughness measurement. Among the several parameters that profilometers can provide, the average roughness Ra [27] (for twodimensional (line) measurements), and roughness Sa (for three dimensional measurements in areas) are usually employed for thermal sciences. The schematic of Figure 1, right side, details a typical two-dimensional profile filled with liquid that represents the channel surface finishing.

Figure 1. Schematic of liquid slug at evaporator zone. Region I indicates local dry-out, II equilibrium film, III evaporating thin film, IV meniscus region, V is the liquid slug and VI as advancing vapor plug.

As show in Figure 1, the heat delivered through the casing wall may encounter different parallel thermal resistances. This depends on the liquid-surface behavior, which, in turn, depends on the roughness. Sensible heat transfer due to the evaporation at liquid film-vapor interface dominates the heat transfer mechanisms during start-up period. Afterwards, nucleate boiling at liquid-solid interface becomes the major mechanism. However, these two phase change mechanisms depend on activate nucleation sites on the internal heated wall surface. On the start-up period, the liquid layer is considered as having a constant thickness 𝛿𝑙, which depends on the working fluid fill volume. The larger the surface roughness (Ra), the larger is the solid-liquid interface area, as shown in Figure 2. On the left side (Figure 2a), a rougher surface is represented while the right side (Figure 2b) depicts a smoother one. For a fixed liquid film thickness, the total thermal resistance is a function of liquid-solid contact area. Before the onset of the first vapor bubble, conduction heat transfer between wall and liquid is observed and all the evaporation happens at the liquid-vapor interface. In this case, the saturation temperature and pressure of the working fluid in the evaporator zone increase. If the heat flux is not high enough, liquid remains static, i.e., with velocity V=0. In this static condition, only the receding 𝜃𝑟 and advancing 𝜃𝑎 angles change, to induce static equilibrium forces. In turn this results in an increase of the advancing radius 𝑅𝑎 and a decrease of receding radius 𝑅𝑟 [28]. At low heat fluxes, the local dry-out (Region I) does not exist when the surface is fully wet. Sensible-evaporative heat transfer mechanism continues to dominate during the period before the bubble formation. Although the extension of Regions I-VI along the tube is highly stochastic when the PHP is fully working, before of the full start-up, the formed bubbles tend to expand at evaporator zone, affecting the liquid thickness and the length of each Figure 1 region. Even in the absence of oscillations, Regions II, III and IV suffer modifications in length and shape. If the temperature difference between the wall (Tw) and the vapor (Tv) increases, the evaporation rate 𝑚 also increases and the length of the film decreases. Adding more heat, there will be a heat flux where the dry-out (Region I) appears [29]. As there is no liquid in the dry out region, Tw increases.

a)

b)

Figure 2. Heat transfer mechanism at liquid film in a same control volume, for the heat transfer across the solid wall and liquid film by conduction before the start-up; finally, evaporation occurs at vapor-film interface. a) Relative roughness surface 𝑅𝑎~𝛿𝑙 ; b) Relative low roughness surface 𝑅𝑎 < 𝛿𝑙

The full start-up of a PHP is characterized by the nucleation boiling phenomena along the internal tube surface (see Figure 3) in the regions where the liquid film is thick enough, i.e., Regions III, IV and V. The strong adhesive forces between liquid and solid at the very thin film region (II), do not allow phase change to happen. The liquid slugs are pushed (oscillate) by the vapor pressure inside the bubbles, which must reach a certain level to overcome friction and other forces over the liquid. Therefore, the oscillations depends on the time necessary for the bubble growth and for the vapor pressure to build up. It is well known in the literature that rougher surfaces provide more cavities, which play the role of nucleation sites (see Figure 3), reducing the superheat temperature (Tw > Tsat 2σ Tsat

+ R𝑐 γlvρv, where Rc is the minimum activated cavity radius), necessary to the onset of vapor bubbles. This is especially true for regions where the liquid thickness is much larger than the roughness cavities (𝛿 ≫ 𝑅𝑎), which, in the present case, is assumed as of a triangular shape, of height of 2Ra, according to [30] (see Figure 3a,b).

a)

b)

Figure 3. Heat transfer mechanism at liquid film. Stage 2 occur when minimum superheating is achieved to bubble growth of embryo contained in an activated cavity. Boiling nucleation occurs and instant pressure increases at evaporator zone. 𝑅𝑐 𝑐𝑎𝑠𝑒 𝑎 > 𝑅𝑐 𝑐𝑎𝑠𝑒 𝑏; 𝑇𝑤 𝑐𝑎𝑠𝑒 𝑎 < 𝑇𝑤 𝑐𝑎𝑠𝑒 𝑏

After the start-up, the thermal behavior of the PHP is highly affected by the presence and distribution of liquid along the evaporator zone. According to [31], high wettability between liquid and tube material helps in the liquid film formation along the device, decreasing the pressure drops. Besides, the liquid film acts as a lubricant, helping liquid slugs to be moved along the serpentines [32]. Furthermore, when a liquid slug is dislocated, a thin liquid film trail is formed on the solid surface due to the wettability [33]. Lastly, this affects the evaporation and the existence of active nucleation sites.

3. Experimental apparatus 3.1. Fabrication procedure PHPs are formed from diffusion bonding of two plates containing semicircle cross section channels manufactured through CNC milling. The milled flat plate contains five U-turns at evaporator zone, resulting in ten parallel channels with diameter of 2.5mm. The PHPs are divided in section A (70x150mm2) and B (138x150mm) respectively (see Figure 4). Section A corresponds to the evaporator plus 1/3 of the adiabatic region while section B contains the remaining 2/3 of the adiabatic region and the condenser. The channels of the hybrid PHP were finished with a Grit N100 sandpaper in section A and N1200 in the section B. On the other hand, the channels of the conventional PHP, were finished with sandpaper Grit N1200 on both A and B sections. All the sanding was performed following ASTM E3-11 Standard [34].

Figure 4. Overall dimensions: 150mmx208mm and material copper. Thickness of 5mm after the diffusion bonding of two plates of 2.5mm each one.

After milling, the PHP contacting surface plates were cleaned, by immersion in a solution of H2SO4 with a concentration of 10% in volume. The planar contacting surfaces (of around 24,000mm2) were sanded in two steps: first a sandpaper Grit N600 was used to eliminate the scratches resulted from the milling process; afterwards, a N1200 Grid sandpaper was used. Good finishing of the contacting surfaces guarantee good diffusion bonding between them. PHPs bonding process was carried out applying the process parameters for copper flat plates, described by [16] and [21] and sketched in Figure 5. Basically it consists on the application of 6.5MPa of mechanical pressure (Figure 5a), at a temperature of 900°C on a high vacuum atmosphere of 5x105 mbar (Figure 5b).

a) Plates layout and PHP assembly

b) Thermal cycle

Figure 5. PHP manufacturing process a) Two plates forming each PHP and schematic of pressure applications with hydraulic press; b) Thermal cycle to PHP diffusion bonding.

3.2. Channel wall parameter characterization Surface roughness is the main parameter used to characterize the channel wall surfaces. To get a appropriated roughness measurements, representative copper specimens were prepared following the same sanding procedure applied to the channel. The device used to measure the surface roughness was Interferometry Analysis Zygo® NewView7300, with 640x480 pixels, which covered an area of 2.828 x 2.121 mm2. Roughness analysis was made using Gwyddion 2.51 Image Analysis Tool. The roughness parameters: maximum height and average roughness, Sz and Sa, respectively, were used to characterize the surface, according to ISO4287 standard. A picture of the surface profile and the resulting measured parameters are shown in Figure 6 and Table 1. GRIT N100

GRIT N1200

Figure 6. 3-D Roughness analysis of copper samples. Interferometry Analysis Zygo® NewView7300, with 640x480 pixels, covering an area of 2.828x2.121mm2. Roughness analysis made with Gwyddion 2.51 Image Analysis Tool. Table 1. Roughness statistical quantities. Sa and Sz analysis according ISO4287 standard. Measurement type Statistical Quantities N100 N1200 Maximum peak height (Sa): 3.36 µm 0.28 µm Sample roughness Maximum height (Sz): 5.01 µm 0.95 µm

3.3. Experimental setup Basically, the experimental setup is composed of a heater that delivers heat to the PHP evaporator and of a cooler (heat exchanger) that removes heat from the condenser (see Figure 7). There is an adiabatic region between the heater and cooler. A programmable power source TDK-Lambda GEN300-17 supplies power to two cartridge electrical resistances (10 mm of diameter and 100 mm of length) embedded on two copper blocks, with a contact area with the evaporator of 5,750 mm2. Each electrical resistance supplies up to 210 W of power. The cooler consists of an aluminum block of 140 mm x 80 mm. performing an area of 11,200 mm2, containing two parallel channels that allow water to flow through it, with controlled inlet temperature, supplied by a thermal bath Lauda Proline RP1845 of 20 [l/min] nominal flow. Thermal grease Omegatherm® 201 is applied at the interfaces between PHP and evaporator and between PHP and condenser interfaces to reduce the contact resistance. The entire test rig is isolated with mineral wool of around 20 mm of thickness. After the diffusion bonding of the PHP, leakage tests are made in an Edwards Spectron 5000 helium leak detector, to guarantee the PHP fabrication quality and vacuum tightness. Temperatures were measured by thirteen calibrated T-type thermocouples, connected to DAQ-NI SCXI-1000 data acquisition module. The calibrated thermocouples present a maximum error of Emax = ± 0.3°C. Thermocouples 1 to 5 are located on condenser zone, 6 to 8 on adiabatic zone and 9 to 13 on evaporator zone, as shown in Figure 7. The flat plate PHPs were discharged and charged, before each experimental test cycle, with the following procedure: extracting the working fluid located at inner channels using a low-vacuum pump, which runs for at least two hours: evacuation of the tube with a

diffusive pump, resulting in high vacuum, of the order of 1x10-6 mbar and, finally, filling the PHP with a controlled amount of distillated, degassed water.

Figure 7. Experimental setup. 1-PHP, 2-Heater, 3-Adiabatic zone, 4-Condenser a) Inlet flow b) Outlet flow, 5-Support.

3.4. Experimental procedure The two PHPs were tested with the condenser temperature set at 20°C, for all the experiments. First, the power applied was increased from 20 W up to 350 W in several power steps. To investigate the effects of hysteresis, the supplied input power was then decreased from 350 W back to 20 W. The elapsed time of each power level was set at 900s, which is the time necessary for the temperature of the evacuated PHP to show a temperature variation of less than 0.3°C/min. Both devices were tested for filling ratios (FR - volume of liquid over internal volume of the PHPs) of 25, 50 and 75% at both vertical (heat-bottom) mode and horizontal positions. A test with FR=0% was performed to identify the thermal performance of the PHPs subject only to conduction heat transfer mechanism. As a next step, tests were made in vertical and horizontal position, under extremely low filling ratio conditions, aiming to determine the filling ratio level beyond which the dryout would appear. For instance, as dryout was not observed for filling ratio of 15% for the tested power levels, the filling ratio was reduced to 10%, in which conditions dryout was detected. Experimental results showed that the PHP with filling ratios of 50% worked at horizontal position. With 25%, the PHP showed random temperature oscillations. This indicates that the device seemed to almost reach operation conditions, but it was not fully successful. For filling ratios of 50%, the device presented proper working conditions. For filling ratios of 75%, the PHP stopped working as a two-phase device, showing a thermal behavior similar to a copper plate transferring heat only by conduction, for all the power levels tested. Thus, the filling ratio of 37.5%, the average value between 25 and 50%, the best performance was observed, with the PHP in full operation, still with relatively small volume of working fluid. Table 2 shows the liquid volume for each tested filling ratio. Table 2. Liquid volume for experimental filling ratios

Filling ratio % 10% 15% 25% 37.5% 50% 75%

Volume [ml] 1.62 2.43 4.05 6.08 8.10 12.15

Volume Uncertainty [ml] 0.06 0.06 0.10 0.10 0.10 0.14

The thermal resistance is defined as the ratio between the difference of the average temperatures of evaporator and condenser and the transferred heat. The maximum thermal resistance uncertainty 𝑈𝑚𝑎𝑥 was obtained by means of propagation of errors, based on the uncertainties of voltage U(V) and current U(I) measurements, as provided by manufacturer (i.e. U(V)=30 mV, U(I)=8.5 mA). The

average temperature difference uncertainty was 𝑈(∆𝑇)=0.07°C. 𝑈𝑚𝑎𝑥 was found to be of 10% at 20 W, decreasing to less than 1% at 350 W. 4. Results and discussion The transient thermal behaviors of the tested PHPs were analyzed, to evaluate the influence of surface roughness of the inner channel walls, at different filling ratios, on the thermal performance of the PHP. The evaporator mean transient temperatures of the homogeneous and hybrid surface finishing PHPs are presented in Figure 8, for the devices working in vertical and Figure 9 and 10, for horizontal positions. From these plots, three operating transitory events are observed: start-up (STU), dry-out (DO) and stop motion (SM). In addition, three operational regimes were observed. The conductive heat transfer (CR), unstable (UR) and stable oscillations (SR) regimes. The CR is detected before the device start-up (STU)), after the dry-out (DO) and whenever the power input is not enough to maintain the oscillation motion. In this last case, the fluid movement stops (SM). The other two regimes, UR and SR, are interrupted only if dry-out (DO) occurs. 4.1. Data analysis of increasing power level from 20W to 350W The first operation regime is the conductive heat transfer (CR). This regime is characterized by uniform temperature readings. Occasionally, fluid presents random isolated temperature oscillations, mainly at horizontal position, due to vapor plugs displacements. These are characterized by high amplitude and low frequency and tend to disappear over time. This regime actually ends with the completion of the start-up time, which is characterized by the onset of the oscillation behavior of the PHP. At vertical position, for the homogeneous and hybrid PHPs, the temperature before the startup increases smoothly [1], with no oscillations, as shown in Figure 8. This figure also shows that the start-up for the homogeneous PHP with FR smaller than 25%, is characterized by a temperature overshoot of around 10°C above the mean temperature at evaporator zone. In most cases, PHPs, in vertical position, do not work at heat loads lower than 40W; only hybrid PHP with FR of 10 and 15% achieved start-up conditions at power inputs of 20W, showing an unstable performance. Start-up depends on the FR, appearing for most cases, between 40 and 60 W of input power. For FR of 25%, start-up happens at 40 W while, for FR of 37.5%, at 60W. For FR of 50%, start-up is observed at 80W and lastly, for FR of 75%, PHPs show an unexpected result: the homogeneous started-up at 80W, while, the hybrid, at 180W. After reaching this point, the temperature starts oscillating, characterizing the full start-up. Unlike of homogeneous device, start-up for hybrid device is characterized for the absence of temperature overshoots. After start-up, the unstable regime (UR) is observed. This stage is characterized by the appearance of temporary stop motion conditions; just before the start of the oscillation motion, an agglomeration of bubbles may take place in the entire evaporator zone, forming a single large coalesced bubble. This makes the working fluid in the PHP to temporarily stop, until the temperature builds up, to overcome this condition. This case is identified as the “stop-over” (SO), also reported in the literature [29]. The evaporator mean temperature increases during the SO, for hybrid PHP with FR of 10% at vertical position, for power inputs between 140 and 180 W, is around 28°C (see Figure 8a). For the hybrid PHP with FR of 15%, the SO temperature increase was of around 35°C (Figure 8b), while for FR of 25%, of around 24°C (Figure 8c). However, the SO temperature increase drops to around 8°C, for hybrid PHP with FR of 37.5% (Figure 8d), and to 20°C (Figure 8f), for hybrid PHP with FR of 75%. SO was observed for the homogeneous PHP with FR of 50%, for heat powers between 180 and 230W, showing a temperature increase of 15°C (Figure 8e). As shown in Figure 8, SO happens more frequently for the hybrid PHP at vertical position. The sustained stable oscillation regime (SR), as reported by [13], is observed after a critical heat flux is applied, when the PHPs achieves a pseudo steady state regime. In this stage, the temperature curves and overall thermal resistance present and asymptotic behavior over time for higher input power levels. The amplitude of the temperature oscillations and the mean temperature level are both reduced. In this regime, the stop-over phenomena disappears. The mean evaporator temperature is

Figure 8. Evaporator transient mean temperatures at vertical position for hybrid and homogeneous PHPs. a) FR10%, b) FR15%, c) FR25%, d) FR37.5%, e) FR50%, f) FR75%. Power inputs from 20W to 350 to 20W. Time of 900 seconds at each power level. (STU) Start-up, (SM) Stop motion, (SO) stop-over, (DO) dry-out.

also less sensitive to power input variations. For FR of 75%, the homogeneous PHP achieves stable oscillation at 180 W, while the hybrid PHP never reaches stable performance. In almost all of the tests, at 180 W and higher input powers, the oscillation motion turned to a stable regime, with the disappearance of the stop-over, and the regime becomes of sustainable stable oscillation. Dry-out zone (DO). This state is observed only at FR 10% at vertical position, for homogeneous PHP. In this condition, the temperatures increase for power inputs higher than 230W (see Figure 8a). 4.2. Data analysis of decreasing power mode from 350 to 20 W The stable regime achieved at 350W was kept while the input power was decreased, up until unstable regime appeared again. Thermal behavior at decreasing power levels is, in most cases, similar to the increasing power level mode, with some transitions between stable and unstable regime, finishing with a stop motion (SM) behavior, when power input is not enough to maintain oscillation motion. At this point the PHP returned to the conductive heat transfer mechanism, observed before the start-up conditions. Although thermal regimes at increasing and decreasing power level modes are similar, it is important to highlight that transitions between regimes can occur at different power levels. This effect is known as thermal hysteresis [35],[36],[37]. As a consequence, the input power at stop motion is different than that at the start-up (i.e. the device stops working at different power input levels than needed for start-up). This phenomenon was analyzed by means of the parameter (∆𝑇STU ― ∆𝑇𝑆𝑀)𝑄, where ∆𝑇STU is the mean temperature difference between evaporator and condenser zones, for a fixed heat load (W) at increasing power mode, and ∆𝑇SM is the same mean temperature difference at decreasing power mode. If (∆𝑇STU ― ∆𝑇𝑆𝑀)𝑄 = 0, the hysteresis does not happen. High hysteresis was observed at hybrid PHP with FR of 10%, which achieved start-up condition at 20W and stopped the oscillations at 140W and at FR of 25%, which started at 40W and stopped at 140W. On the homogeneous PHP, for FR of 10%, dry-out conditions were achieved at 350W, possibly by the absence of liquid at evaporator zone. However, the device started to work again at 180W for the power input at decreasing mode. Overall signs of hysteresis were more pronounced on the hybrid PHP with FRs lower than 25%. In these cases, the oscillations stopped at high power levels for the devices operating at decreasing power supply. Therefore, hysteresis is an important phenomenon that deserves careful analysis. Table 3 summarizes the regimes of the PHP operating at vertical position. For FR smaller than 25% and horizontal position (see Figure 9), thermal performances of both homogeneous and hybrid PHPs are similar to a conductive copper plate. This was observed for all tested power ranges, with the appearance of some random oscillations, as if the system attempted to start. These oscillations increased with the increase in FR. In the case of hybrid PHP with FR of 25%, high temperature amplitude oscillations of approximately 60°C are observed, even though the sustained oscillations are not achieved. In the case of FR of 37.5 and 50 %, (see Figure 10) at horizontal position, conductive heat transfer mechanism dominated in the first power levels. Nevertheless, start-up condition were eventually achieved. For homogeneous PHP with FR of 37.5%, the start-up happened at 140W, On the other hand, for the same FR, the hybrid PHP started at 180 W. Furthermore, for hybrid PHP with FR of 50%, start-up is observed at 350 W, while for homogeneous PHP this happens at 140W. At horizontal position, the thermal behavior of both PHPs with FR 75% is similar to the conductive copper plate and is not shown in this paper. On the other hand, for PHPs with FR of 37.5%, the difference in the temperature amplitudes was of 25 °C for the homogeneous PHP and of 15 °C for the hybrid PHP. For the PHPs with FR 50%, this difference was of 25°C for both devices.

Table 3. Regime at vertical position. CR: conduction heat transfer; UR: unstable regime; SR: stable regime; DO: dry-out. Hysteresis observation: (Y=Yes, N=not).

CR

20

40

60

80

100

140

180

230

290

350

290

230

180

SR

5.1

4.1

CR

UR

CR

CR SR

3.2

UR

Y

UR

UR

2.5

0.7

0.4

2

q'' [W/cm ]

UR

CR

Hom

CR

SR

CR

Hyb

75%

SR UR

UR

Y Y N

CR

Y

0.4

Hom.

UR

Y

0.7

Hyb

CR

3.2

CR

Y

SR

4.1

Hom

N Y

CR

UR

5.1

CR

CR

CR SR

6.2

Hyb

Y N

1.1

UR

UR

1.4

CR

SR

1.8

CR

Hom

UR

2.5

Hyb

CR

SR

Hystereis Y

SR

SR

1.8

50%

UR UR

1.4

37.5%

CR

CR

DO

UR

1.1

25%

140

SR

Hyb Hom

SR

CR

Hom

15%

100

UR

Hyb

10%

80

60

Type

40

FR

20

POWER [W]

a) b) c) Figure 9. Evaporator transient mean temperature at horizontal position for hybrid and homogeneous PHPs. Random oscillations at: a) 140 and 180 W for FR10%, b) 140 and 230 W for FR15%, c) 140, 230, 290 and 350 for FR25%. Power inputs from 20W to 350W.

Table 4 shows a map only of the PHPs that were able to operate at horizontal position. Both PHPs needed more power input to achieve start-up as compared to vertical position bottom heat mode. Furthermore, the evaporator mean temperature varies over a broad range, following the results presented by [35]. The PHPs with other FRs were not able to achieve the sustained oscillation motion regime. According to [14], horizontal position operation is possible if the number of turns is enough to guarantee the internal perturbations and if the heat load is enough to provide the necessary pressure to maintain the oscillations. For high FR, oscillations were not observed due to low vapor available and the low pressure drop, as reported by [38]. The operational regimes explained above differs from previous works that reports a thermal crisis at high power levels with a partial dry-out under different channels, without achieving a total device stop motion [35], [36], [39]. Experiments showed that stop-over produces a temporary full stop motion, similar to the dry-out regime. However, by varying the applied heat load, this instability can be overcome and the oscillation motion characteristic of the stable regime is achieved.

Figure 10. Evaporator transient mean temperature at horizontal position for hybrid and homogeneous PHPs. a) FR of 37.5, b) FR50%. Power inputs from 20W to 350 to 20W. Time of 900 seconds at each power level. (STU) Start-up, (SM) Stop motion. Table 4. Regime at horizontal position. CR: conduction heat transfer; UR: unstable regime; ST: stable regime. Hysteresis observation: (Y=Yes, N=not).

20

40

60

Y

0.4

Y

0.7

Y

CR

1.1

CR

1.4

1.8

6.2

5.1

SR

4.1

3.2

80

100

140

180

230

290

350

290

230

180

140 2.5

1.4

1.1

0.7

0.4

2

SR UR

Y

CR

CR CR

Hom

q''[W/cm ]

100

80

SR

Hyb

50

Hystereis

CR

2.5

CR

SR

3.2

Hom

UR

4.1

CR

5.1

Hyb

1.8

37.5

60

Type

40

FR

20

POWER [W]

4.3. Thermal resistance The overall thermal resistance 𝑅𝑡 of the PHPs is determined as the ratio between the temperature difference between the evaporator and condenser regions and the power transferred (𝑄). Thermal resistance allows for the evaluation of the lumped thermal performance of heat pipes. Considering 𝑇𝑒 as the mean value of the temperatures obtained from thermocouples T9 to T13, located at the evaporator zone and 𝑇𝑐 is the mean condenser temperature from thermocouples T1 to T5, at the condenser zone, the thermal resistance can be determined as: 𝑅𝑡 =

𝑇𝑒 ― 𝑇𝑐 𝑄

(1)

Thermal resistance for evacuated PHP, (FR of 0%) was measured to be 0.42±0.01°C/W. In this condition, pure conduction is the dominant heat transfer mechanism. The thermal resistance was measured in the same experimental conditions for both PHPs with different filling ratios, as a function of the power input. The heat losses were calculated to be less than 2%. Figure 11 shows the PHP thermal resistances at vertical position, bottom heat mode. From this figure it is noted that the hybrid PHP presented an early start-up and lower thermal resistances for FRs lower than 50%. The hybrid PHP also showed an increase in the thermal resistance, for all

cases where the filling ratio was less than 25%, at power levels between 140W and 180W. This coincides with the observed stop-over phenomena. The rise in thermal resistance is shown in Figure 11a,b,c. The thermal resistance increase was expected, as the complete liquid stop motion causes the evaporator temperature to increase. The homogeneous PHP with FR of 75% has around half of the thermal resistance of the hybrid PHP for the same FR, after the start-up. In this case, the channel surface roughness hinders the working fluid biphasic movement, creating elevated pressure drops, which resist to the oscillations. Thus, the advantages gained from the increased roughness is surpassed.

Figure 11. Thermal resistance at different filling ratios, as a function of power input at vertical position bottom heat mode.

Figure 12 presents the thermal resistance of the PHPs operating in horizontal position. The thermal resistance decreased for power inputs between 140 and 180 W. Actually, as shown in Figure 9a,b,c, even though the PHPs did not achieve sustained regime in these conditions, random temperature oscillations were observed. The temperature peaks were able to reduce slightly the thermal resistance of the devices and, as the hybrid PHP creates strongest oscillations, their thermal performance tend to be better. It is possible to see the drastic reduction of thermal resistance at hybrid PHP at power levels higher than 230 W for FR 37.5%, and higher than 290W, for FR 50%.

Figure 12. Thermal resistance for FR 37.5% and 50% at horizontal position, as a function of power input.

The variation of thermal resistance (%𝑉𝑎𝑟) is defined as the difference, in percent, between the hybrid PHP and homogeneous thermal resistances, as follows: %𝑉𝑎𝑟 =

(

𝑅𝑡, ℎ𝑦𝑏 ― 𝑅𝑡,ℎ𝑜𝑚 𝑅𝑡,ℎ𝑜𝑚

)

∗ 100

(2)

𝑊

Plots of the variation of thermal resistances as a function of the power input are shown in Figure 13. In this figure, a data point above the 0% line means that the homogeneous PHP has a better performance, while below the line, that the hybrid works better. In general, hybrid PHP presents lower thermal resistances for FR up to 50%, for both testing positions, and for heat loads lower than 140W. For the homogeneous PHP tested at horizontal position for power levels up to 180W, the thermal resistances variation ranges to 10%, but beyond 180W this difference increases, to about 20%. At vertical position, the difference is much more sensitive to the volume of the filling ratio. In general, the hybrid PHP works better than the homogeneous PHP for FR lower than 50%, both at vertical and horizontal position (as also shown in Figures 11 and 12), with the exception of the FR of 75%. A stop-over behavior area is highlighted in the plots and it happens at lower power inputs for the vertical than for the horizontal positions. The thermal resistance of the hybrid PHP, when the stable regime at vertical position was achieved (at 230W), was around 40% lower than that of the homogeneous PHP.

Figure 13. Comparative variation of thermal resistance only for FR that achieved start-up conditions: a) Horizontal, b) Vertical.

In general, the hybrid PHP works better, with exception for the operation conditions when stopover appeared. Asymptotic behaviors of thermal resistances are observed for power levels higher than 180W, at vertical position. One should note that both PHPs did not work at FR of 10%, 15 and 25% at horizontal position. However, at horizontal position and for FR of 37.5 and 50%, both PHPs achieved start-up conditions. It can be observed that the thermal resistance decreases for the vertical orientation.

It is observed that the filling ratio has a significant effect, as broadly reported in the literature [40], [41]. Moreover, roughness also has a significant effect, as demonstrated experimentally, especially if the FR is lower than 25% and higher than 75%. The combination of effects: orientation, FR and roughness plays relevant role for power inputs lower than 100W, affecting strongly the start-up behavior of the device. The literature reports a wider stable operation regime that depends of each experimental conditions, including FR varying between 20 and 80% and different tilt angles[14]. In the present case (modified channel surface roughness), stable operation regimes were found in PHPs with FR ranging between 37.5% and 50% at vertical and horizontal position. 5. Summary and conclusions Two flat plate PHP with different internal surface roughness and 2.5mm of inner diameter were tested. Their thermal response for the transient temperature behavior and thermal resistances, were compared among themselves, for the devices operating with several filling ratios and two orientations: vertical and horizontal. From this comparison, the following observations could be made:  The thermal behavior of the PHP can be divided in three main operational regimes: conduction, where the heat transfer mechanism depends mainly of the solid material; unstable, where stop-over instability appears and the thermal resistance starts to decrease; stable, where the stop-over is not observed and the temperature oscillations are sustained. Regimes are separated by three states: startup (between conductive regime and oscillating mode); dry-out (observed only for low filling ratios and high power levels) and stop motion (the device stop operating). The start-up conditions were reached at lower heat fluxes in the hybrid device. The hybrid PHP, which internal channel surfaces were finished with Grit N100 at evaporator zone and with 1200 at condenser zone, presented lower thermal resistance at vertical position for most of the FR tested (10, 15, 25, 37.5, and 50%), for power levels between 20 and 140 W and 230 and 350 W, when compared to homogeneous Grit N1200 finishing PHP devices. Stop-over was the most visible instability state, happening at power levels between 140 and 180 W, mainly for the hybrid PHP; thus, the stop-over instability continues to be a challenge to be overcome.  Higher roughness at evaporator zone increases the number of available nucleation sites, in turn, reducing the start-up onset. However, higher roughness increases the shear stresses and hinder the oscillations for high filling ratios (FR 75%). The hybrid PHP showed higher thermal resistance and needed more heat power input to achieve the start-up. Thermal hysteresis was observed for low filling ratios; this fact highlights the importance of the operation conditions in the minimum power needed for the start-up and stop motion.  In the horizontal position, start-up was achieved only for FR 37.5 and 50%. At horizontal position, the oscillations showed ampler amplitudes and the heat fluxes necessary to achieve sustained oscillations were also bigger. The thermal resistances were more pronounced, when compared with those obtained for the device operating in vertical position, even when the full activation was reached. In both cases, the hybrid PHP showed lower thermal resistances, at heat fluxes higher than 230 W.  The experimental results presented in this paper highlight the importance of the effect of the channel surface roughness in the thermal performance of PHPs. Diffusion bonding fabrication is an interesting process which allows channel surface modifications. It enable a mixture of several surface improvement techniques, e.g. alternating capillary channels and/or the use of more channels, keeping the same external area; besides, roughness modifications increased the thermal instability depicted as stop-over, which can be reduced by means of grooves at evaporator zone. Further works needs to explore this solution. Roughness variation due to the thermal cycling to which the PHPs were subjected during their fabrication may be considered and studied in the future. In addition, further works are needed to explore the roughness that would lead to better surface

finishing. Besides, visualization of phase change phenomena occurring at evaporator zone is suggested, to identify differences in the bubble flow dynamics, due to channel surface finishing. Acknowledgements: The authors wish to acknowledge the help of all the members of the Heat Pipe Laboratory at the Federal University of Santa Catarina. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES) - Finance Code 001. Nomenclature CR D DO Emax FR Hom Hyb I 𝑚 P PHP Q R Ra Rc Rt Sa SM SO SR STU Sz T 𝑇 U UR V V Var W

Conductive regime Diameter [m] Dryout Maximum error Filling ratio [%] homogeneous hybrid Input current [A] Evaporation rate [kg/s] Pressure Pulsating heat pipe heat load [W] Radius [m] Mean surface roughness [µm] Cavity radius [µm] Thermal resistance [°C/W] Mean surface roughness [µm] Stop motion Stop-over Stable regime [-] Start-up Maximum height surface roughness [µm] Temperature [°C] Mean temperature [°C] Standard uncertainty Unstable regime [-] Input voltage [V] Velocity [m/s] percent variation [%] Power [W]

Subscripts a c e l lv max r s S t

Advancing condenser zone evaporator zone liquid liquid-vapor maximum Receding stress Stop [-] total

sat Stu w v

saturation Start-up [-] wall vapor

Greek γlv ∆𝑃 ∆𝑇 𝛿 𝛿𝑙 𝛿𝑜 𝜃 ρ 𝜎 𝜏𝑠

Enthalpy of vaporization [J/kg-°C] Pressure difference [Pa] Mean temperature difference [°] Evaporating film region thickness [µm] Liquid film [µm] Equilibrium film region thickness [µm] Contact angle [°] Density [kg/m3] Surface tension [N/m] Shear stress [Pa]

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Luis Alonso Betancur Arboleda Assistant researcher, Heat Pipes Laboratory, Federal University of Santa Catarina R. Eng. Agronômico Andrei Cristian Ferreira, s/n - LABTUCAL / Departamento de Eng. Mecânica, BL.A3 - Terceiro Andar - Trindade, Florianópolis - SC, 88040-900 Monday, october 14 2018 Dear Executive Editor Prof. William Worek, I wish to submit an original research article entitled “Experimental Study of Channel Roughness Effect in Diffusion Bonded Pulsating Heat Pipes” for consideration in "Applied Thermal Engineering" Journal. I confirm that this work is original and has not been published elsewhere, nor is it currently under consideration for publication elsewhere. The authors declare that there is no conflict of interest to disclose. Please address all correspondence [email protected].

concerning

Thank you for your consideration of this manuscript. Sincerely,

_________________________________________ Luis Alonso Betancur Arboleda

this

manuscript

to

me

at

Highlights 

This research reports the roughness effect in diffusion bonded pulsating heat pipes.



The device with higher roughness at evaporator zone increases thermal performance.



Stop-over instability appears on the device with increased thermal performance.