# High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters

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## Abstract

**:**

## 1. Introduction

_{i}is the inner diameter of the OHP, ρ

_{l}and ρ

_{v}are the corresponding densities of liquid and vapor, σ is the surface tension coefficient, and g is the gravity acceleration. Accordingly, several researchers [12,30] suggested the inner diameter of the OHP should be satisfied as

_{i}is suggested within the following range [31]:

_{i}= 1.0 and 2.0 mm, which indicated the better thermal performance of the OHP with bigger D

_{i}. In addition, via the comparison between the thermal resistance of two OHPs with different inner diameters, Yang et al. [33,34] found that the thermal performance of the OHP with D

_{i}= 1.0 mm was decreased by about 10% relative to that with D

_{i}= 2.0 mm. In addition, the shape of the cross section of the OHP also affects its thermal performance under the same hydraulic diameter. The effects of inner diameter on the thermal performance of OHP might be opposite for different working fluid, which is indicated by the experimental results by Rittidech et al. [35,36]. Particularly, based on an experimental test on the open-loop OHPs, Saha et al. [37] found that when compared with the OHP with D

_{i}= 0.9 mm, the OHP with D

_{i}= 1.5 mm demonstrated worse thermal performance, which is different from the conclusions by Charoensawan et al. [32] and Yang et al. [33,34].

_{i}= 1.0, 2.0, 3.0 mm is investigated and compared to reveal the fundamental effects of the inner diameter. In addition, the motions of working fluid, distributions of vapor and liquid phases as well as the temperature distribution in the condenser are clarified and analyzed, in an effort to elucidate the relationship between the motions of working fluid and the heat transfer characteristics in the OHPs.

## 2. Experimental Setup

_{o}= 6.0 mm and inner diameters D

_{i}= 1.0, 2.0, 3.0 mm, respectively. As depicted in Figure 1b, the OHP with the dimensions of 400 mm × 185 mm is set vertically and heated at the bottom, which includes evaporator, adiabatic section and condenser with corresponding lengths of 100 mm, 25 mm and 275 mm. Before the experiment, the OHP is baked at 100 °C, evacuated to be 4.0 × 10

^{−4}torr for 8 h by a vacuum pump, and then filled with methanol as the working fluid. The filling ratio is maintained at φ = 47% in this work. For the evaporator, the Ni-Cr wire with diameter of 0.25 mm is wrapped on the outer tube wall to supply the uniform heating load for the OHP. The whole evaporator and adiabatic section are embedded into the insulation box stuffed with the aluminum silicate insulation fibers, so as to ensure the relative error of heating load Q within 4.9%. The whole experiment is conducted in an environment with a constant temperature of 15 ± 0.5 °C, and thus the condenser of OHP can be cooled by the forced convection of the surrounding air via a cooling fan. By using an NEC TH9260 infrared camera (NEC Corporation, Tokyo, Japan) and the corresponding software, the infrared thermal images of the condenser during the operation of OHP are monitored, recorded and analyzed. The infrared camera possesses operation wavelength of 8–14 μm and noise equivalent temperature difference (NETD) of ±0.08 °C, as well as thermal image resolution of 640 × 480 pixels. The emissivity of the condenser surface is corrected before the experiment via comparing temperature signals measured by the infrared camera and the K-type thermal couple with a measuring error of ±0.1 °C. By checking the emissivity of condensation section, the deviation between the tested result of the condenser temperature and its real value via the infrared camera is less than 2.3 °C. The temperature of evaporator is measured by the thermocouples (diameter of 0.25 mm, OMEGA K-type with measuring error of ±0.1 °C, Omega Engineering, Santa Ana, CA, USA) fixed at the bottom of the evaporator, as marked in Figure 1b. The evaporator temperature are read and recorded by an Agilent 34970A data acquisition switch unit (Agilent Technologies, Santa Clara, CA, USA) with 6.5-digit accuracy, which has a maximum relative error of 0.5%.

## 3. Results and Discussions

#### 3.1. Fluid Flow Motions inside OHP

_{t}represents the percentage of duration for a certain fluid flow motion with respect to the total statistical duration. It can be seen that when Q is very small, the driving pressure inside the OHP is limited, which only triggers small oscillation of working fluids. With the increasing heating load, the driving pressure inside the OHP rises to overcome the flow resistance between the evaporator and condenser or even the adjacent U-turns, which induces the bulk oscillation of working fluid. At this time, the small and bulk oscillations appear intermittently in the OHP, and the duration of bulk oscillation is gradually increased when the heating load (i.e., the driving pressure in the OHP) further rises, as depicted in (i) and (ii) of Figure 4b. By further raising the heating load, the region of bulk oscillation is expanded to multiple U-turns, which eventually produces the circulation of working fluid (see (iii) of Figure 4b). Finally, the small oscillation and bulk oscillation of working fluid disappear sequentially under a large heating load, and the pure circulation of working fluid is achieved in the OHP (see Figure 4a and (iv) of Figure 4b). Note that, as shown by the comparison among the flow motions of working fluid in the OHPs with different inner diameters under the same heating load (see (ii), (v) and (vi) in Figure 4b), with respect to the OHP with D

_{i}= 2.0 mm, OHP with D

_{i}= 1.0 mm has larger frictional flow resistance inside, and thus the fluid flow motions under small driving pressure (e.g., small heating load) appears more easily (e.g., small oscillation in (ii), (v) and (vi) of Figure 4b). On the other hand, although the frictional flow resistance is lower in the OHP with D

_{i}= 3.0 mm, the resistance of capillary hysteresis is higher, and the vapor-liquid meniscus of vapor slugs is less rigid, which weakens the necessary ‘bubble pumping’ action [33,34] for the momentum transfer of working fluid inside the OHPs. Consequently, the percentage of oscillation motions is larger than that in the OHP with D

_{i}= 2.0 mm (e.g., small oscillation in (ii), (v) and (vi) of Figure 4b).

#### 3.2. General Flow Patterns in OHP

#### 3.2.1. Bubbly Flow

_{i}= 1 mm restricts the nucleate boiling in the evaporator [46], resulting in the fewer dispersed bubbles in the bubbly flow than that under D

_{i}= 2 mm, 3 mm. In addition, the dispersed bubbles in the OHP with D

_{i}= 1 mm are hardly mixed due to the confinement, and always flow with the main stream in turn.

#### 3.2.2. Slug Flow

_{i}= 1 mm is mainly formed by the self-growth of dispersed bubbles and breakup of very long slugs rather than the coalescence of dispersed bubbles, which is different from that under the inner diameter of 2 mm and 3 mm.

#### 3.2.3. Annular Flow

_{i}= 1 mm. These irregular waves easily cause the break of the continuous vapor core in the annular flow via the formation of the liquid bridges, which can induce the transition from the annular flow to the slug flow. Therefore, the stability of annular flow deteriorates with the decreasing inner diameter of the OHPs.

#### 3.3. Flow Pattern Evolutions in Evaporator and Condenser

#### 3.3.1. Flow Pattern Evolutions in Evaporator

_{i}= 1 mm, rather than the coalescence among them in the evaporators with diameter of 2 mm and 3 mm. Furthermore, less dispersed bubbles are produced by the nucleate boiling with the decreasing inner diameter, which further explains the characteristics of bubbly flow in the OHP with D

_{i}= 1 mm as discussed in the above section.

#### 3.3.2. Flow Pattern Evolutions in the Condenser

_{i}= 1 mm, forming more short slugs than that under D

_{i}= 2 mm, 3 mm.

_{i}) at the condensers of the OHPs with different D

_{i}under S-O and B-O motions of working fluid at Q = 90 W, where L is the real length of bubble/slug (see Figure 5). Herein, the percentage of bubble/slug length, P

_{i}, is introduced to quantitatively represent the length distribution of bubbles/slugs

_{j}is the bubble/slug number within a certain range of dimensionless size, and n

_{t}is the total number of bubbles/slugs in a statistic duration of 10 s. As shown, the proportion of dispersed bubble (L/D

_{i}< 1) increases as the tube diameter increases, implying that the nucleate boiling at the evaporator with bigger inner diameter produces more dispersed bubbles flowing into the condenser and thus increases their proportion there. Compared with dispersed bubbles, as the inner diameter increases, the proportion of short slugs (1 ≤ L/D

_{i}< 10) experiences a decrease. As mentioned above, this is mainly attributed to the drop of capillary instability in the OHP, which reduces the probability of breakup from the long slugs to the shorter ones. Moreover, this decreasing capillary instability with increasing inner diameter also induces the larger proportion of long slugs (100 ≤ L/D

_{i}) under D

_{i}= 2 mm than that at D

_{i}= 1 mm. On the other hand, enlarging the inner diameter of the OHP also amplifies the absolute distance between vapor slugs, which decreases the probability of coalescences among vapor slugs into long vapor slugs, resulting in smaller proportion of long slugs (100 ≤ L/D

_{i}) under D

_{i}= 3 mm than that at D

_{i}= 2 mm.

#### 3.4. Thermal Performance

_{1}~T

_{5}as shown in Figure 1b, and ${\overline{T}}_{\mathrm{c}}$ is the average temperature of condenser computed by averaging the temperature values of all pixel points on the infrared thermal images of the condenser. In addition, in order to represent the effect of inner diameter on the thermal performance of the OHP, the dimensionless inner diameter of the OHP, D

_{i}*, is defined as

_{i}is the inner diameter of the OHP, g is the gravity acceleration, and Δρ and σ are the difference of density between liquid and vapor and surface tension coefficient at (T

_{e-r}+ T

_{c-r})/2, respectively. Herein, T

_{e-r}and T

_{c-r}are the reference temperatures of the evaporator and condenser, which are correspondingly defined as the highest operating temperature of the evaporator T

_{e-r}= 123.8 °C and ambient temperature of the condenser T

_{c-r}= 15 °C. Actually, D

_{i}* is another expression form of the Bond number, which can characterize the ratio between the surface tension and gravity. As indicated in Figure 9, under the small oscillation of working fluid, the heat exchanged between the hot and cold ends of the OHP mainly relies on the heat conduction of working fluid and tube body, resulting in a slight decrease of R with the increasing heating load. When the bulk oscillation of working fluid occurs, the heat and mass transfer in the OHPs are significantly improved, leading to the most apparent decrease in R versus the increasing heating load (e.g., the changing of R within Q = 60–120 W under D

_{i}* = 1.31 (D

_{i}= 2 mm) in Figure 9). After the appearance of working fluid circulation, the capabilities of heat and mass transfer in the OHPs are further enhanced to be a steady level due to a certain amount of filled working fluid, when the thermal resistance of the OHPs gradually drops to be a steady value (e.g., the changing of R within Q = 110–200 W under D

_{i}* = 1.31 (D

_{i}= 2 mm) in Figure 9). Note that the dry-out of working fluid will emerge in the evaporator, represented by the sudden growing-up of the thermal resistance (e.g., Q = 160 W under D

_{i}* = 0.65 (D

_{i}= 1 mm) and Q = 220 W under D

_{i}* = 1.99 (D

_{i}= 2 mm) in Figure 9).

_{i}* = 0.65 (D

_{i}= 1 mm), due to the large frictional flow resistance. Furthermore, even under the same mode of working fluid motions, the large frictional flow resistance under D

_{i}* = 0.65 (D

_{i}= 1 mm) also reduces the heat and mass transfer rate into the OHP. Therefore, the OHP with D

_{i}* = 0.65 (D

_{i}= 1 mm) has the highest thermal resistance relative to those under D

_{i}* = 1.31 (D

_{i}= 2 mm) and D

_{i}* = 1.99 (D

_{i}= 3 mm). In addition, owing to the hardest supplement of working fluid for the nucleate boiling in the evaporator under the least amount of filled working fluid, the dry-out of working fluid occurs most easily in the OHP with D

_{i}* = 0.65 (D

_{i}= 1 mm). Compared with the OHP with D

_{i}* = 1.31 (D

_{i}= 2 mm), the gravity plays a more important role in the OHP with D

_{i}* = 1.99 (D

_{i}= 3 mm), producing the thermosyphon effect under the small oscillation of working fluid which facilitates the backflow of working fluid into the evaporator and enhances the heat and mass transfer [34]. Accordingly, at the small heating load (Q ≤ 40 W), thermal resistance under D

_{i}* = 1.99 (D

_{i}= 3 mm) is a little smaller than that under D

_{i}* = 1.31 (D

_{i}= 2 mm). However, under the bulk oscillation depending on the momentum exchange of working fluid, the thermal resistance under D

_{i}* = 1.99 (D

_{i}= 3 mm) turns out to be slightly larger than that under D

_{i}* = 1.31 (D

_{i}= 2 mm) (50 W≤ Q ≤ 100 W). It must be attributed to the decrease of rigidity on the vapor-liquid meniscus of vapor slugs, which weakens the necessary ‘bubble pumping’ effect for the exchange of working fluid in the OHPs and reduces the efficiency of heat and mass transfer.

## 4. Conclusions

_{i}= 1 mm than that under D

_{i}= 2 and 3 mm.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 1.**Experimental apparatus: (

**a**) schematic of experimental setup; (

**b**) schematic of experimental oscillating heat pipe; and (

**c**) cross section geometry of A–A in inset (

**b**).

**Figure 2.**Fluid flow motions in the oscillating heat pipe (OHP) and corresponding experimental images as well as infrared thermal images of the condenser: (

**a**) schematic of fluid flow motions; (

**b**) snapshots of different fluid flow motions in a typical U-turn of condenser; and (

**c**) infrared thermal images of the condenser.

**Figure 3.**Vertical temperature distribution of condenser under different quasi-steady operation states corresponding to Figure 2c (6#-7# U-turn): (

**a**) small oscillation (Q = 30 W); (

**b**) big oscillation (Q = 70 W); and (

**c**) circulation (Q = 140 W).

**Figure 4.**Variation of fluid flow motions with increasing heating load: (

**a**) schematic of change in fluid flow motions with increasing heating load; and (

**b**) time series of fluid flow motions and their corresponding duration fractions under different heating loads and inner diameters. Fluid flow motions mode index: 0: S-O; 1: B-O; and 2: C.

**Figure 5.**Flow patterns occurring in the OHPs with different diameters. Flow patterns indexes: B: bubbly flow; S: slug flow; A: annular flow.

**Figure 6.**Evolution of flow pattern in the evaporators of the OHPs with different inner diameters (Q ~ 100 W).

**Figure 7.**Evolution of flow pattern in the vertical tube at the condenser of the OHPs with different inner diameters: (

**a**) snapshots of flow pattern evolution under D

_{i}= 1mm; (

**b**) snapshots of flow pattern evolution under D

_{i}= 2mm; and (

**c**) snapshots of flow pattern evolution under D

_{i}= 3mm.

**Figure 8.**Distribution of bubbles/slugs length at the condenser in the OHPs with different inner diameters under quasi-steady S-O and B-O fluid motion: (

**a**) D

_{i}= 1mm under Q = 90W; (

**b**) D

_{i}= 2mm under Q = 90W; and (

**c**) D

_{i}= 3mm under Q = 90W.

**Figure 9.**Variation of thermal resistance and fluid flow motions versus increasing heating load under different inner diameters of the OHPs.

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## Share and Cite

**MDPI and ACS Style**

Liu, X.; Sun, Q.; Zhang, C.; Wu, L.
High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters. *Appl. Sci.* **2016**, *6*, 321.
https://doi.org/10.3390/app6110321

**AMA Style**

Liu X, Sun Q, Zhang C, Wu L.
High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters. *Applied Sciences*. 2016; 6(11):321.
https://doi.org/10.3390/app6110321

**Chicago/Turabian Style**

Liu, Xiangdong, Qing Sun, Chengbin Zhang, and Liangyu Wu.
2016. "High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters" *Applied Sciences* 6, no. 11: 321.
https://doi.org/10.3390/app6110321