Experimental investigations on operating characteristics of a closed loop pulsating heat pipe

Abstract

The operating mechanism of the pulsating heat pipe (PHP) is not well understood and the present technology cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand the operation regimes of the PHP through experimental investigations. A series of experiments were conducted on a closed loop PHP with 5 turns made of copper capillary tube of 2 mm in inner diameter. Two different working fluids viz. ethanol and acetone were employed. The operating characteristics were studied for the variation of heat input, filling ratio (FR) and inclination angle of the tested device. The results strongly demonstrate the effect of the filling ratio of the working fluid on the operational stability and heat transfer capability of the device. Important insight into the operational characteristics of PHP has been obtained.

Keywords

closed loop pulsating heat pipe / thermal performance / operation limit / thermography

Introduction

As a novel two-phase passive heat transfer device, the qualitative as well as quantitative investigations of the pulsating heat pipe (PHP) are emerging at a rapid pace. The PHP design variants being proposed and studied have the potential to meet present high heat-flux electronics cooling [ 1– 4], heat recovery [ 5, 6], air preheating and drying [ 7, 8], to name only a few.

In recent years, a spurt of research activity has been conducted to understand the operation mechanism of the PHP and intertwined parameters affecting the operating characteristics of the PHP, which primarily concentrate on visualization experimental investigation to better understand start-up, stable operation and operation limit of the PHP by recognizing the flow pattern of working fluid in PHP; heat-transfer performance experimental study of the effect of operational parameters (such as heating or cooling mode, tilt angle), thermo-physical characteristics of working fluid and geometrical characteristics on stable operation of the PHP; and construction of mathematical model and suggestion of governing equations for motion and heat transfer to predict the PHP operation.

It is now known that there are 3 flow patterns (bubbly flow, slug flow and annular flow) during start-up and operation of the PHP. With the increase of heat input, the bubbly flow turns into the slug flow, resulting in the oscillatory flow in the PHP. The oscillatory flow then turns into the circulating flow with the annular flow being the main flow as the heat input further increases [ 9– 12]. Qualitative analysis can be made on the thermal state and flow pattern of the working fluid in the PHP by thermal imaging technology combined with temperature measurement [ 13– 15].

The heat transfer performance of the PHP depends on many intertwined factors. Gravity is a key factor to affect the start-up and operation of the PHP. The number of U turns can greatly reduce the effect of gravity. The PHP is usually under stable operation at a tilt angle of 70°–90°. The thermo-physical properties of fluid affects the optimal heat transfer ability of the PHP. A better heat transfer performance can be gained at a higher heat load with water as the working fluid while at a lower heat load with ethanol or R123 as working fluid. There exists an optimum filling ratio, which is influenced by the working fluid and operating conditions. Most studies indicate that the optimum filling rate is 40%–60%. Some reports indicate that a lower filling ratio (30%, 35%) is better for heat transfer under stable operation [ 16– 23].

Some analysis models based on fluid mechanics, heat transfer and thermodynamics are established for making analysis on start-up or stable operation conditions of PHP combined with the gained experimental phenomenon [ 24– 28]. Other investigations are based on the artificial neural network or the chaos theory. Statistical analysis is conducted on experimental data to reveal the PHP operation mechanism [ 29– 31].

Most research efforts significantly increase the understanding of the phenomena and parameters that govern the thermal performance of the PHP [ 32]. However, many unresolved issues still exist. Continued exploration should be executed to realize comprehensive theory for reliable design of PHP.

In view of the technological importance of the startup and thermal performance of the PHP, a series of thermo-fluid dynamic investigations were conducted in this paper on a closed loop pulsating heat pipe (CLPHP) with 5 turns, where the thermocouples were integrated in the tube. The local measurement of temperature, together with thermal images, provided the operating characteristics of the PHP.

Experimental set-up and procedure

The tested CLPHP is made of a copper capillary tube, the inner diameter of the CLPHP is determined by Eq. (1) [ 33].

(1) \mathrm{0.7}\sqrt{{\displaystyle \frac{\sigma }{\left({\rho }_{\mathrm{l}\mathrm{i}\mathrm{q}}-{\rho }_{\mathrm{v}\mathrm{a}\mathrm{p}}\right)g}}}\le D\le \mathrm{1.8}\sqrt{{\displaystyle \frac{\sigma }{\left({\rho }_{\mathrm{l}\mathrm{i}\mathrm{q}}-{\rho }_{\mathrm{v}\mathrm{a}\mathrm{q}}\right)g}}},

where σ is the surface tension (10^-3 N/m), ρ_liq is the working fluid density of the liquid (kg/m³), ρ_vap is the working fluid density of the vapor (kg/m³), g is the gravity accelerate (N/kg), and D is the inner diameter of capillary tube (m).

As ethanol and acetone are employed as the working fluid of the tested CLPHP, the inner diameter ranges from 1.2 mm to 3.5 mm. Then a copper tube with an inner diameter of 2-mm is selected to make the tested CLPHP.

Two ways are usually adopted to test the thermal performance of the PHP: controlling heat flux input and condenser temperature; and controlling the evaporator and condenser temperature [ 34, 35]. The former strategy was adopted in the present experiment. The experimental set-up shown in Fig. 1 consists of the tested CLPHP, a power supply system for heating, a cooling jacket and a data acquisition system. The CLPHP was connected to the vacuum gauge and filling valve by a four-connecting junction union. The tap water flowing through the cooling jacket maintained the thermal condition in the condenser section of the CLPHP. The mass flow rate of the cooling water could be measured by an electronic scale. 15 “T” type thermocouples were fixed on the wall of the CLPHP, 5 of which measured the temperature variations in the evaporation section, 5 of which measured the temperature variations in the condensation section, and 5 of which measured the temperature variations in the middle section. Another 2 thermocouples were fixed in the outlet ant inlet of the cooling jacket, measuring the temperature difference of the cooling water. All thermocouples were connected to the data logger (Agilent, 34970A), and the temperature data was recorded and saved in a PC. An infrared thermal imager was employed for qualitative investigations on the operation characteristics of the tested CLPHP.

Each experiment is conducted with the following procedure:

1) Preliminary operations:

①All peripheral devices (PC, data logger, voltage regulator, voltmeter, and ammeter) are switched on. The power supply is standby (no electric power is provided to the evaporator). The cooling jacket is filled with cooling water.

②The tested PHP is connected to the vacuum pump (Varian, DS102) through a sealing valve. When the desired vacuum level is reached, the valve is closed and the vacuum stability inside the PHP is checked again by the vacuum gauge located at left branch of the four-connecting junction union.

③A 10mL syringe is filled with the working fluid and is connected to the filling valve. Then the filling valve is slowly turned on in such a way that the working fluid can enter the tested PHP. When the desired volume of fluid, corresponding to a filling ratio, is inside the PHP, the filling valve is turned off.

2) Test procedure and primary operations:

①The temperature data acquisition system is enabled.

②The power supply voltage is increased in order to provide the desired amount of heat-input for the PHP start-up. The desired heat input level is maintained until the measured temperature achieves constancy.

③Regulating cooling water flow rate to ensure that temperature difference between the outlet and inlet of cooling jacket is no less than 3°C to maintain the certainty of heat transfer measurement. When the quasi-steady-state is established, switch the valve in order that the cooling water flows into the bucket above the electronic scale for flow rate recording.

④Representative infrared thermography which reflects the internal flow pattern regimes to some extant is stored. All temperature measurements are also simultaneously recorded by the data logger.

⑤Steps 2 to 4 are repeated till the operation limit is reached. When dry-out phenomenon is getting initiated, it is manifested with a decrease in the overall thermal performance of PHP; a sharp increase in the evaporator temperature and therefore a corresponding increase in thermal resistance is noticed.

⑥A series of such experiments are repeated when the working fluid (ethanol, acetone) is changed.

⑦A series of experimental investigations are conducted at different inclination angles of the tested PHP under stable operation condition from vertical position to horizontal position (90°, 60°, 45°, 30°, 0°).

The experimental investigations on operating characteristics of the CLPHP are conducted with the variation of heat input (adjusting power supply) and filling ratio (35%, 53%, and 70%) of the tested CLPHP with ethanol and acetone employed as the working fluid respectively, as listed in Table 1.

Two indexes are employed for evaluating the thermal performance of the tested CLPHP:

1)‚The heat transfer of the tested CLPHP, which is the heat removed by condenser jacket water, is calculated by

(2) Q=\dot{m}{C}_{\mathrm{p}}\left({\overline{T}}_{\mathrm{c,out}}-{\overline{T}}_{\mathrm{c},\mathrm{i}\mathrm{n}}\right),

where \dot{m} is the mass flow rate of cooling water, C_{\mathrm{p}} is the specific heat value of water, {\overline{T}}_{\mathrm{c,out}} and {\overline{T}}_{\mathrm{c,in}} are respectively the average temperature of the outlet and inlet of cooling jacket, {\overline{T}}_{\mathrm{c,out}} is the average temperature of T_{\mathrm{17}} , and {\overline{T}}_{\mathrm{c,in}} is the average temperature of T_{\mathrm{16}} .

2) The thermal resistance is defined as

(3) R={\displaystyle \frac{\left({\overline{T}}_{\mathrm{e}}-{\overline{T}}_{\mathrm{c}}\right)}{Q}},

where {\overline{T}}_{\mathrm{e}} is the average value of measured temperatures in the evaporator section, {\overline{T}}_{\mathrm{e}}={\displaystyle \frac{1}{5}}{\displaystyle \sum _{i=11}^{15}\overline{T_i}} ; and {\overline{T}}_{\mathrm{c}} is the average value of measured temperatures in the condenser section, {\overline{T}}_{\mathrm{c}}={\displaystyle \frac{1}{5}}{\displaystyle \sum _{i=1}^5\overline{T_i}} .

Results and discussion

In the experiments mentioned in Table 1, the flow rate through cooling jacket for working fluid condensation in the tested CLPHP is almost constant. Temperature difference between the outlet and inlet of the cooling jacket reflects the heat transferred from the evaporator to the condenser. Based on temperature distribution and thermal images on the tested CLPHP, operating characteristics of CLPHP can be compressively understood.

Thermal performance evaluation

The thermal performance evaluations of the performed experiments are demonstrated in Fig. 2. It is observed from Fig. 2 that the thermal resistance of the tested CLPHP with acetone is lower than that with ethanol when the heat transfer is below 100 W at 35% FR and 53% FR or below 120 W at 70% FR. This indicates that the thermal resistance decreases till the heat-transfer quantity increases to approximately 100 W at 35% FR and 53% FR or the heat-transfer quantity arrives at approximately 120 W at 70% FR.

It also can be seen from Fig. 2(a) and (b) that heat-transfer limit exits in the tested CLPHP. When acetone was employed as the working fluid, the thermal resistance increases obviously after the heat-transfer quantity reaches 100 W at 35% FR and 53% FR. The results are similar as presented by Yang et al. [ 36]. The operation limit of the tested CLPHP will be further explained in Section 3.2.

Operation limit

It can be seen from Fig. 3 that the temperature difference between the outlet and inlet of cooling jacket (T17–T16) increases slightly as the heat input increases from 81 W to 196 W, while the temperature difference between the evaporator section and the condenser section increases sharply. (The temperature measurement of the evaporator section or the condenser section approximately presents a consistent fluctuations. T1 reflects the temperature of the evaporator section and T11 reflects that in the condenser section.) A corresponding increase in the thermal resistance can be noticed when the heat input increases from 134 W to 196 W.

It is shown in representative infrared thermography that the branches of the evaporator section are obviously brighter than that of the condenser section. The possible flow patterns can be estimated that the slug flow is mainly in the branches of the condenser zone whereas the annular flow is mainly in the branches of the evaporator zone. Based on the fact that the temperature at the evaporator occasionally soars with the increase of the heat input, and frequently soars after the heat input reaches 196 W, it can be estimated that when the CLPHP with a low FR operating at sufficient heat input, the working fluid flow in the evaporator is annual flow and local dry-out phenomena should appear.

It can be seen from Fig. 4 that the tested PHP operated stably before the 2840th second and after the 3000th second. During the test period between the 2840th second and the 3000th second, the evaporator temperature increased sharply and the tested PHP operated with a decrease in the overall thermal performance.

Before the 2840th second, the representative infrared thermography indicated that the cooler branch (in dark color) and the hotter branch (in bright color) flickered alternately; After the 3000th second, the representative infrared thermography showed that the hotter branch (in bright color) and the cooler branch (in dark color) flickered alternately; Between the 2840th second and the 3000th second, all branches of the tested PHP flickered in bright color.

It can be estimated that the working fluid in the tested PHP circulates with directional flow, as is noted by white arrows in the thermography reflecting stable operation in Fig. 4.

Judging from the temperature and the corresponding thermal images in Test-d, the flow pattern of the working fluid can be estimated. When the tested CLPHP operated stably, the slug flow and the annual flow appeared in the adjacent parallel channels, while the working fluid in the tested CLPHP circulated directionally; when the tested PHP worked unstably, the working fluid oscillated in the PHP while the slug flow appeared in the condensation section and the annual flow appeared in the evaporation section. If the heat input increased continuously, the evaporation section would be filled with the working fluid vapor while the condensation section would be filled with the liquid working fluid and the heat-transfer deteriorated, as described in Refs. [ 37, 38].

From Fig. 3 and Fig. 4, the conclusion can be drawn that operation limit may appear in CLPHP with low FR under sufficient heat input working condition.

Influence of inclination on operation

The influence of inclination on operation is illustrated in Figs. 5 and 6. It is noticed from Figs. 5 and 6 that there is no significant variation of heat-transfer capability in the tested CLPHP when the inclination angle is adjusted from 90° to 60°.

It can be seen from Fig. 5 that the heat-transfer of the tested CLPHP does not get worse until the inclination angle is adjusted from 30° to 0°. Figure 6 reveals that the local dry-out phenomena appeared occasionally when the inclination angle reached 45° and appeared tempestuously after the inclination angle reached 60°.

When the tested CLPHP operated horizontally, the temperature at the evaporation section increased sharply and temperature difference between inlet and outlet disappeared, which indicated that the CLPHP stopped working.

Conclusions

Experiments were conducted to achieve a better understanding of the operating characteristics of the CLPHP with 5 turns employing ethanol and acetone as the working fluid. Based on the results obtained from the experiments, the following conclusions can be reached:

A lower FR results in a higher heat-transfer performance in the CLPHP under stable operating state.

The FR and heat-input are important factors governing the heat-transfer capability and the operation limit of the CLPHP. When the CLPHP with a lower FR works at a high heat-input condition, the dry out phenomena appears and the heat-transfer deteriorates.

The thermal properties of the working fluid affect the heat-transfer performance of the CLPHP. The working fluid with a low latent heat such an acetone results in a lower thermal resistance in stable operation of the CLPHP.

Gravity cannot be ignored for stable operation control of the CLPHP. The tested CLPHP cannot operate horizontally.

Additional investigations should be conducted to understand the operation regimes of the CLPHP through visualization experiments combined with the results obtained in this study.

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