Operation Of Pulsating Heat Pipes Research Articles

Detailed visualization experiments on the start-up process and stable operation of pulsating heat pipes: Effects of dividing boards

To broaden the applicable orientation for the existing closed loop pulsating heat pipe (CLPHP) with a relatively large tube (internal diameter of 6 mm), this work fabricated a novel quartz CLPHP inserted with a dividing board in the straight section of each tube. The dividing board forms two semi-circles with an effective hydraulic diameter (Dh) of 3.09 mm. This method amplifies the surface tension effect with almost no change in the mass inventory of working fluid. Through a series of visualization experiments, the effect of filling ratio (FR) on the pulsation in the boarded model are first revealed by comparing with the original non-boarded CLPHP under the vertical and the horizontal orientations. The four longitudinal corners generated by dividing board are found not only accelerated the drain down of the liquid slugs but beneficial to the onset of nucleate boiling, prompting the threshold of pulsation motion for 10 - 20 W. Also, the visualization results for the vertical orientation show that the sealing between the dividing boards and the inner tube had a great impact on the performance and heat transfer behavior. The thermal resistance can be improved by 22 - 43 % without leakage between the semi-circles. Under the horizontal orientation, the boarded CLPHP with only three number of turns can be incited with pulsation motion successfully. The boarded design provides additional vapor channels and avoids the liquid slug trains being pushed away from the evaporator during the initial heating period. Also, the un-balanced capillary force due to the hydraulic diameter difference can sustain subsequent pulsation motion. However, the success under only FR = 50 % indicates that an appropriate FR was also necessary.

Operating mechanism of pulsating heat pipe with different wettability from the perspective of thermo-hydrodynamic characteristics of vapor–liquid interface region

Pulsating heat pipe (PHP) is considered to be one of the most promising efficient heat sinks in electronic thermal management for good cooling capacity and compact structure. Understanding the complex operating mechanism of PHP from various angles is crucial for advancing its large-scale application. Among various factors, the thermo-hydrodynamic characteristics of the vapor–liquid interface region have a critical impact on the operation of PHP. Based on this, the transient numerical simulations were conducted in this work on PHPs with 20° and 150° contact angles (hydrophilic and hydrophobic, respectively), and the influence of different thermo-hydrodynamic characteristics of the vapor–liquid interface region on the operating mechanism of PHPs was analyzed. The results found that the presence of a widely spreading liquid film at the interface of hydrophilic PHP significantly changed its thermo-hydrodynamic characteristics compared to hydrophobic PHP. As a consequence, the hydrophilic PHP and hydrophobic PHP presented better heat transfer performance in the evaporation section and condensation section, respectively. Under low heat inputs, with regular vapor–liquid distribution in the interface region, the hydrophobic PHP presented better startup and heat transfer performance due to better hydrodynamic behavior caused by more easily formed pressure gradients. Under high heat inputs, the hydrophilic PHP showed significantly better heat transfer performance due to the intense evaporation and continuous spreading of new liquid film in the interface region, which also played a key role in stronger anti-dry-out performance.

Detailed visualization experiments on the start-up process and stable operation of pulsating heat pipes: Effects of internal diameter

In the present work, detailed flow characteristics in glass pulsating heat pipes (PHPs) are pioneeringly organized into category charts based on careful visualization during the start-up and the steady operational periods, respectively. Two IDs of 6 mm and 3 mm are studied and compared. The larger ID of 6 mm weakens the effect of surface tension that all the water sinks to the bottom after the filling. Whereas, sections of liquid slugs are formed initially in the 3 mm-ID PHP. Pulsation is observed to operate on and off under low values of heat input (Qin), and gradually develop into steady oscillation with increasing Qin. In the low-Qin start-up period, different pulsation motions exist in separate tubes, which is associated with the initial distribution of liquid slugs. Geyser pulsation, which tends to occur for ultra-long liquid slugs, recurs irregularly in the 6 mm-ID PHP. For the 3 mm-ID PHP, the smaller ID and the lighter weight of working fluid intensify the surface tension effect against gravity that the liquid slug trains can be easily driven around, preventing the geyser pulsation. During the operation period, in either the 6 mm-ID or the 3 mm-ID PHP, continuous up-and-down pulsations switch between the liquid and vapor states with increasing Qin. The critical Qin for the operational period with continuous pulsation is related to the filling ratio: 70 W for FR = 35 %, 60 W for FR = 50 %, and 40 W for FR = 65 % in the 6 mm-ID PHP.

On the interaction between liquid slug and vapor bubble in the chaotic operation of pulsating heat pipe

A pulsating heat pipe has shown promising results in heat transfer enhancement for several industrial applications. Despite having multiple advantages in the design and physics-based parameters, its applications are still limited due to the lack of understanding of the dynamics of the chaotic interaction of the liquid slug and vapor bubble. The randomly distributed liquid slug and vapor bubble start oscillating upon applying the heat at the evaporator section. A transition from the self-sustained oscillatory to chaotic operation in the pulsating heat pipe has been observed in various experimental studies. However, there are only a few works explaining the chaotic interaction of liquid slugs and vapor bubbles. We present the analysis of the oscillatory behavior of the position of the liquid slug in the pulsating heat pipe using a nonlinear mathematical model. The identification of various operating regions in the parameter space using the bifurcation analysis reveals the presence of a route to chaos by the period-doubling bifurcation. The fast Fourier transform of the temporal evolutions confirms the presence of the period doubling phenomena. Furthermore, the Lyapunov exponent and the correlation dimension are used to detect chaos and quantify the dimension of the chaotic attractor, respectively. Moreover, the Hurst exponent is used to determine the persistency of the oscillations, which indicates that the oscillations are weakly persistent in the chaotic regime. The transition to the chaotic regime is analyzed by period doubling, and a change in the frequency beyond period-doubling shows a gradual shift to the chaotic operation.

A review on start-up characteristics of the pulsating heat pipe

Pulsating heat pipe (PHP) is a two-phase heat transfer device, and the start-up characteristics plays a crucial role in its practical application and stable operating performance. This paper introduced the start-up principle of PHP and reviewed the influences of the cross-sectional shape, the physical property parameters and the external field action (i.e., magnetic field, ultrasonic wave etc.) on the starting characteristics of PHP. Based on the results of present literature, the pressure distributions in PHP with non-circular channel section or irregular channel are uneven, which easily form an oscillation cycle motion and is beneficial to the efficient operation of PHP. Simultaneously, the fluids with high thermal conductivity, low dynamic viscosity and low surface tension (such as nano-fluid, deionised water, etc.) are more conductive to starting characteristics of PHP. In addition, reviewed studies reported that as an active heat transfer enhancement method, external field has great potential in terms of enhancing heat transfer and controlling the heat transfer process.

An improved correlation to predict the heat transfer in pulsating heat pipes over increased range of fluid-filling ratios and operating inclinations

Previous correlations that have been used to predict the heat transfer performance of pulsating heat pipes (PHPs) offer limited thermal predictions within a narrow range of fluid-filling ratios and PHP inclinations. In this paper, a novel semi-empirical correlation with improved scope is proposed, with an increased range of fluid-filling ratios and PHP inclinations. The proposed correlation employs the dimensionless numbers governing the thermo-hydrodynamic operation of PHPs, and achieved ±30 % accuracy when predicting selected experimental data, showing reasonably good agreement. Unlike previous correlations, the new correlation can be used for different working fluids, geometrical aspect ratios, and heat loads. A comprehensive assessment of the relative significance of the correlation parameters on the total heat transfer performance is discussed. The new correlation with its flexible application range is expected to assist in faster and more enhanced thermal predictions as interest in PHPs grows.

Small-Sized Pulsating Heat Pipes/Oscillating Heat Pipes with Low Thermal Resistance and High Heat Transport Capability

Electronics (particularly power electronics) are the core element in many energy-related applications. Due to the increasing power density of electronic parts, the demands on thermal management solutions have risen considerably. As a novel passive and highly efficient cooling technology, pulsating heat pipes (PHPs) can transfer heat away from critical hotspots. In this work, we present two types of small and compact PHPs with footprints of 50 × 100 mm2, thicknesses of 2 and 2.5 mm and with high fluid channel density, optimized for cooling electronic parts with high power densities. The characterization of these PHPs was carried out with a strong relation to practical applications, revealing excellent thermal properties. The thermal resistance was found to be up to 90% lower than that of a comparable solid copper plate. Thus, a hot part with defined heating power would remain at a much lower temperature level and, for the same heater temperature, a much larger heating power could be applied. Moreover, the dependence of PHP operation and thermal properties on water and air cooling, condenser area size and orientation is examined. Under some test configurations, dryout conditions are observed which could be avoided by choosing an appropriate size for the fluid channels, heater and condenser.

Experimental study on a criterion for normal operation of pulsating heat pipes in a horizontal orientation

In this study, a criterion for normal operation of micro pulsating heat pipes (MPHPs) even in a horizontal orientation is developed through experimental and theoretical investigations. Using MEMS techniques, a ten-turn closed-loop MPHP with overall dimensions of 50 × 33.5 × 1.7 mm3 is fabricated. The rectangular micro-channels engraved on a silicon wafer have a width of 1 mm and a depth of 0.5 mm. Ethanol is used as the working fluid and the filling ratio is 47% on the average. To allow flow visualization, a glass wafer is bonded to the silicon wafer. The temporal variations in the temperature and pressure of two neighboring vapor plugs in the MPHP are measured using micro-thermocouples and pressure transducers. The flow behavior in the MPHP is recorded using a high-speed camera and fully synchronized in real time with the measured temperature and pressure data. To find a criterion for normal operation of PHPs, the MPHP is treated as a mass-spring-damper system, and it is postulated that the transition from (to) a stopover to (from) a large-amplitude oscillation in the MPHP occurs with negative (positive) damping. This postulate is experimentally confirmed with the MPHP operating in a horizontal orientation. Finally, a figure of merit for normal operation of PHPs is proposed. The ratio of the minimum thermal resistance in a vertical orientation to that in a horizontal orientation (Rth, V, min/Rth, H, min) is proportional to (Dh/Leff)2, the square of the ratio of the hydraulic diameter to the effective length of PHPs. For PHPs filled with ethanol, it is experimentally confirmed that Dh/Leff has to be greater than approximately 0.03 for normal operation without any stopovers in a horizontal orientation.

Testing and Modeling of the Dynamic Response Characteristics of Pulsating Heat Pipes during the Start-up Process

Pulsating heat pipes (PHPs) are two-phase heat transfer heat pipes with high heat transfer capability and simple structure. Heating power is an important factor that affects the start-up response characteristics of PHPs. The operational characteristics during the start-up and stable operating stages were studied through experiments, and the corresponding dynamic response model under a specified heating power was established based on experimental data and flow pattern in the tube. The starting time, starting temperature, and dynamic response characteristic parameters at a certain heating power were calculated by the dynamic response model. The response characteristics of working fluid during the stable operation of PHPs were deduced based on the dynamic response curve of PHPs during the non-operational and stable operation stages. The response characteristics of PHPs for the step effect (given heating power) were quantitatively described by amplification factor K and time constant τ, thereby presenting the basis for the study on heat and mass transfer mechanisms of PHPs from non-operational to steady operation stage. Results showed that the minimum thermal resistance and the minimum time constant of the PHP are approximately 0.28 °C/W and 75, respectively, obtained at a heating power of 160 W. Moreover, these results indicated that the dynamic response of PHPs demonstrates a favourable performance and rapidly reaches another stable working state when their heat transfer performance is stable. However, the dynamic response time constant of pure fluids decreases when the quantity of the liquid working fluid in the PHP decreases with the increase in heating power.

Thermal performance of a meter-scale horizontal nitrogen Pulsating Heat Pipe

Cooling dry superconducting magnets using cryocoolers as a cold source is becoming a standard. Due to their simple operation, compact configuration, lightness, and thermal performance, Pulsating Heat Pipes (PHP) are good candidates, as two-phase thermal links, for these kind of systems. Initially developed to cool small electronic devices at ambient temperature, PHP have been also studied, in the cryogenic field, using N2, He, Ne and H2 as working fluids. In this paper, we present the thermal performances of a 1 m long horizontal PHP made of 36 stainless steel parallel tubes. The tube internal diameter is 1.5 mm, close to the critical diameter (∼1.7 mm) to maintain capillary forces necessary to the PHP operation. Both, the evaporator and condenser sections are copper made and are separated by an adiabatic section. This PHP has been studied using N2 as working fluid with the condenser temperature maintained at 75 K. Several pressure and temperature sensors placed in the three different sections allow monitoring the thermodynamic behavior of the PHP. Tests have been performed increasing and varying the input power as well as keeping it constant during long periods of time. The maximum equivalent thermal conductivity measured is 85 kW/m·K. This system can transfer a maximum heat power of 25 W before reaching its operation limit.

Evaporation of a liquid film deposited on a capillary heated tube: Experimental analysis by infrared thermography of its thermal footprint

Evaporation of thin film deposited downstream menisci of slug flows is of major importance in the two-phase heat transfer phenomena occurring inside micro/mini-channels or in pulsating heat pipes. The aim of this study is consequently to analyze, by means of experimental infrared thermography, the heat transfers involved in the motion of a semi-infinite slug flow (one liquid slug followed by one single vapour bubble) in a heated capillary copper tube. Between the liquid and vapour phases the meniscus deposits a thin liquid film at the wall; this film brings about intense heat transfer at the wall by evaporation, leading to a highly significant decrease of external wall temperature. This phenomenon is better understood using infrared temperature measurements, which help to analyze the heat transfer characteristics present during evaporation. Given the highly transient nature of the process, the thickness of the deposited film is a parameter of major importance. After validation of the measurement method with tests involving no fluid or only liquid flow, the experimental results, with water being used as a working fluid, are compared to those of a numerical thermal model of which the parameters are well-known. The results drawn from the model rather accurately predict those obtained in the experiments and facilitate further analysis by presenting, for example, the evolution of liquid film thickness or heat flux density in conjunction with the wall temperature fields. It has been found that the thin film length is a first-order function of the initial thickness of the deposited film, the meniscus velocity, and the heat flux density at the wall, including the release of stored heat energy in the wall that is caused by the temperature difference before and after passage of the meniscus. This study represents a step towards better understanding the local physical phenomena governing the operation of pulsating heat pipes.

Taylor bubble-train flows and heat transfer in the context of Pulsating Heat Pipes

Understanding the performance of Pulsating Heat Pipes (PHPs) requires spatio-temporally coupled, flow and heat transfer information during the self-sustained thermally driven flow of oscillating Taylor bubbles. Detailed local hydrodynamic characteristics are needed to predict its thermal performance, which has remained elusive. Net heat transfer in PHP is contributed by (a) pulsating/oscillating flow (b) distribution of different liquid slugs and bubbles, and, (c) phase-change process; however, its contribution is minimal. In fact, the former two flow conditions are largely responsible for heat transfer in PHPs; such flow conditions can be generated without phase-change and can also be studied independently to observe their explicit effects on PHP heat transfer. With this motivation, systematic experimental investigation of heat transfer is performed during (a) isolated Taylor bubble flow (b) continuous Taylor bubble flow and (c) pulsating Taylor bubble flow, at various frequencies (1 Hz to 3 Hz, as applicable for PHPs) inside a heated square mini-channel of cross-section size 3 mm × 3 mm. This study clearly reveals important insights into the PHP operation. Oscillating Taylor bubbles create significant disturbances in their wake which leads to local augmentation of sensible heat transfer. The implications of bubble length, wake characteristics, oscillating frequency and bubble slip velocity on the heat transfer augmentation and, in turn on thermal performance of PHPs can be clearly delineated from this study. The study also brings out the nuances in the estimation of true bubble slip under time varying Taylor bubble flows.

Influence of Heat Input, Working Fluid and Evacuation Level on the Performance of Pulsating Heat Pipe

An experimental study on pulsating heat pipe (PHP) is presented in this work. A closed loop PHP with a single U turn is fabricated and tested. The transient and steady state experiments are conducted and operating temperatures are measured. The experiments are carried out for different working fluids, heat input and for different evacuation levels. The derived parameters include thermal resistance and heat transfer coefficient of PHP. The results of these experiments show an intermittent motion of the working fluid at lower heat input. The temperature difference between evaporator and condenser at steady state is found lower for acetone compared to water, ethanol and methanol. Lower value of thermal resistance and higher value of heat transfer coefficient are observed in case of acetone compared to water, ethanol and methanol. Lower values of temperature difference between evaporator and condenser and thermal resistance and higher value of heat transfer coefficient are observed at atmospheric conditions of operation of PHP compared to evacuation conditions. The Power Spectral Density Analysis is also carried out on the results of these experiments using FFT technique to analyse the pulsating motion of the fluid in a PHP. In the Power Spectral Density analysis, the frequency distribution of temperature variation in PHP was observed over a wider range, signifying the periodic motion in the fluid flow of the liquid slug and vapour plug. This characteristic frequency corresponded to the characteristic time for a couple of adjacent vapour plug and liquid slug passing through a specific local wall surface in a PHP.

THERMOFLUID DYNAMIC STUDY OF FLAT-PLATE CLOSED-LOOP PULSATING HEAT PIPES

Meandering-tube pulsating heat pipes (PHPs) have already foundsome applications in cooling power/microelectroniccomponents. Reliable experimental data for PHPs is very limited,especially in the wake of the ongoing miniaturization process andefforts to combine electronic components and heat spreaders onthe chip level. In fact, the basic operation of PHPs is far frombeing fully understood, and there are no available design tools forgiven microelectronics cooling requirements.A logical next step for further applications of PHPs in microelectronics cooling is to design integral structures, i.e., PHPs as an integral part of thermal spreader/substrates, having typical dimensions as applicable for multichip modules or printed circuit boards (PCBs). In this article, experimental results for flat-plate, closed-loop PHP structures (rectangular and circular, Dhyd ≈ 2.0 mm) which are integrally machined into an aluminum substrate (115 × 36 × 3 mm3) and covered by a glass plate for visualization are presented. Working fluids employed are water and ethanol. To verify certain observed phenomena, another PHP setup is made consisting of 10 glass tubes (ID 2.0 mm) connected by copper U-turns. As input parameters, fill rate, heat load, and tilt angle are varied. Axial temperature distribution is recorded with thermocouples. By means of a high-speed video camera and a thermo-camera, the evaporation/condensation processes and the individual liquid–vapor plug characteristics are studied. The results enable explanations for operational characteristic and performance limits, which are influenced by capillary resistance, gravity, and thermofluid dynamic effects. Preliminary design rules are also discussed.