High-speed Flow Visualization Research Articles

High Speed Imaging Reveals a Close Resemblance Between Drop Impacts on a Solid Surface and the Interaction of a Cold Droplet Placed Inside a Hot Liquid

The phenomenon of drop impacts on a solid surface and the interaction of a cold droplet with a hot liquid pool are both of great relevance in many practical and engineering situations. Most of the reported research in the literature on these phenomena focus either on the impact of a droplet on a solid surface or a liquid surface and there are hardly any study on the interaction of a stationary cold droplet with a hot pool of liquid. This study reports high speed flow visualization of a drop impact on a Teflon-coated solid surface at different liquid properties and impact Reynolds number. Simultaneously, high speed imaging of the interaction of a cold droplet with the hot olive oil pool has also been carried out with a high intensity continuous light source. The images show a striking resemblance between the flow dynamics of the two phenomena. A comparison between the two phenomena is made and suggestions are made for the possible underlying physics.

Local wall temperature mapping during flow boiling in a transparent microchannel

In this study the local two-phase heat transfer coefficients were obtained using wall temperature measurements and fluid saturation temperatures in a rectangular high-aspect-ratio (a=WchHch=22) polydimethylsiloxane (PDMS) microchannel with a hydraulic diameter of 192 μm. The experiments used FC-72 liquid with a mass flux of 7.37 kg m−2s−1 and various heat fluxes ranging between 3.34 and 61.95 kW m−2. Fluid saturation temperatures were determined by interpolating pressure measurements obtained with integrated silicon ceramic-based pressure sensors located near the inlet and outlet of the microchannel. The hydrodynamic and flow boiling characteristics of the microchannel were monitored using high frequency and high spatial resolution infrared thermography, with heat transfer coefficients obtained as a function of axial position, lateral position and time and at the inlet, middle and outlet sections of the microchannel. This enabled the effect of heat flux on local temperature, flow boiling heat transfer coefficient distribution and two-phase pressure drop to be determined. These results suggest that the two-phase heat transfer coefficient does not increase monotonically with the heat flux, and actually decreases under certain conditions. This work identifies that the heat transfer coefficient depends on the range of the heat flux and is correlated with vapour-liquid dynamics and liquid film thinning resulting in suspected dryout, observed simultaneously with the temperature measurements. The simultaneous application of high speed thermography and flow visualization has enabled uniquely detailed information to be obtained that is useful to a more detailed understanding of two-phase flow and bubble dynamics.

Experimental investigation of flow boiling in single minichannels with low mass velocities

Flow boiling characteristics in a single minichannel with the aspect ratio of 2 (1.8 mm width and 0.9 mm height) are studied experimentally. Experiments are conducted for different values of the wall heat flux (in the range of 204.7–431.3 kW/m2) and low mass fluxes (G = 20, 40 and 60 kg m−2 s−1) at a constant inlet temperature of deionized water (approximately 70 °C). Effects of the heat and mass fluxes on the local two phase heat transfer coefficient and the total pressure drop are examined. A considerable attention is also paid to performing high speed flow visualization study to have qualitative information about the flow physics. The heat transfer coefficient shows an irregular trend with increasing heat flux or the vapor quality, while it increases with increasing mass flux. On the other hand, the pressure drop increases with an increase in the heat flux or in the exit vapor quality. For the lowest mass flux value considered, temperature jump or the critical heat flux (CHF) condition is shown to occur at higher heating powers.

Mitigation of impinging tones using central protrusion

This article demonstrates that a simple modification such as installing a protrusion on an impingement plate can lead to reduction of jet impingement tones. This experimental work compares the acoustic variations in jets impinging on a flat plate, and plates with a central protrusion. The key strategy here is the modification of the stagnation flow region brought about by the introduction of the central protrusion. It is observed that the toneless acoustic power is almost the same for jet impinging on the plate with and without the central protrusion. It is also found that the protrusion is more effective in reducing tones in supersonic impinging jets than the tones in subsonic impinging jets. However, there is a slight increase in acoustic power when the jet impinges on a larger protrusion (greater than jet diameter) at high nozzle pressure ratios. Further, high speed flow visualization is carried out to relate the shock oscillations with the tonal frequencies. The noise source location is estimated by performing FFT on the flow visualization images. The acoustic and optical measurements are compared in terms of tonal frequency and found to be in agreement with each other.

A comparative study of experimental flow boiling heat transfer and pressure drop characteristics in porous-wall microchannel heat sink

In this work, we proposed a porous-wall (PW) microchannel heat sink, in which micro pin fin arrays were fabricated on sidewalls of rectangular microchannels by MEMS (Microelectrical Mechanical System) technique. High speed flow visualizations were performed simultaneously with heat transfer and pressure drop measurements to investigate the flow boiling characteristics of PW microchannel heat sink. Conventional rectangular (Rec) microchannel heat sink was also explored together as a comparison. Experiments were carried out with pure acetone liquid at inlet temperature of 30 °C, mass flux from 255 kg/(m2·s) to 843 kg/(m2·s), heat flux from 4 W/cm2 to 110 W/cm2 and the maximum vapor quality at the outlet of the channel was 0.88. Experimental results demonstrated that the PW microchannels reduce wall superheat of onset of nucleate boiling (ONB) and improve critical heat flux (CHF) compared to the Rec microchannels. Moreover, the PW microchannels show significant heat transfer enhancement, pressure drop reduction and mitigation of two-phase flow instability. The porous walls provide numerous nucleation sites and the intensive pin fins arrangements introduce significant wicking effect to maintain the liquid rewetting, which contribute to the above notable flow boiling enhancement.

Experimental investigation of frequency and amplitude of density wave oscillations in vertical upflow boiling

Historically, study of two-phase flow instabilities has been arguably one of the most challenging endeavors in heat transfer literature due to the wide range of instabilities systems can manifest depending on differences in operating conditions and flow geometry. This study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.

High-frequency thermal-fluidic characterization of dynamic microchannel flow boiling instabilities: Part 1 – Rapid-bubble-growth instability at the onset of boiling

Dynamic flow boiling instabilities are studied experimentally in a single, 500 µm-diameter glass microchannel subjected to a uniform heat flux. Fluid flow is driven through the microchannel in an open-loop test facility by maintaining a constant pressure difference between a pressurized upstream reservoir and a reservoir at the exit that is open to the ambient; the working fluid is HFE-7100. This hydrodynamic boundary condition resembles that of an individual channel in a parallel-channel heat sink where the channel mass flux can vary in time. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. Part 1 of this study investigates the rapid-bubble-growth instability at the onset of boiling; the effect of flow inertia and inlet liquid subcooling is assessed. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified; this instability is shown to cause flow reversal and can result in large temperature spikes. Low flow inertia exacerbates the rapid-bubble-growth instability by starving the heated channel of liquid replenishment for longer durations and results in severe temperature increases. In the case of high flow inertia or high inlet liquid subcooling, flow reversal is still observed at the onset of boiling, but results in a minimal wall temperature rise because liquid quickly replenishes the heated channel. A companion paper (Part 2) investigates the effect of flow inertia, inlet liquid subcooling, as well as heat flux on the thermal-fluidic oscillations during time-periodic flow boiling that follows the initial incipience at the onset of boiling considered here.

High-frequency thermal-fluidic characterization of dynamic microchannel flow boiling instabilities: Part 2 – Impact of operating conditions on instability type and severity

Dynamic instabilities during flow boiling in a uniformly heated microchannel are investigated. The focus of this Part 2 of the study is on the effect of operating conditions on the instability type and the resulting time-periodic hydrodynamic and thermal oscillations, which have been established after the initial boiling incipience event. Part 1 of this study investigated the rapid-bubble-growth instability at the onset of boiling in the same experimental facility. Fluid is driven through the single 500 μm-diameter glass microchannel by maintaining a constant pressure difference between a pressurized upstream reservoir and a reservoir downstream that is open to the ambient, so as to resemble the hydrodynamic boundary conditions of an individual channel in a parallel-channel heat sink. Simultaneous high-frequency measurement of pressure drop, mass flux, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior. The effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities, and the pressure drop instability. A spectral analysis of the time-periodic data is performed to determine the characteristic oscillation frequencies. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations due to a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity.

Schlieren photography on freely flying hawkmoth.

The aerodynamic force on flying insects results from the vortical flow structures that vary both spatially and temporally throughout flight. Due to these complexities and the inherent difficulties in studying flying insects in a natural setting, a complete picture of the vortical flow has been difficult to obtain experimentally. In this paper, Schlieren, a widely used technique for highspeed flow visualization, was adapted to capture the vortex structures around freely flying hawkmoth (Manduca). Flow features such as leading-edge vortex, trailing-edge vortex, as well as the full vortex system in the wake were visualized directly. Quantification of the flow from the Schlieren images was then obtained by applying a physics-based optical flow method, extending the potential applications of the method to further studies of flying insects.

A comparative study of flow boiling HFE-7100 in silicon nanowire and plainwall microchannels

Extensive experimental investigations along with high speed visualizations have been performed to assess the flow boiling characteristics in Silicon Nanowire (SiNW) microchannels. Experiments have been also performed in Plainwall microchannels to compare their performances with SiNW configurations. HFE-7100 has been used as the working fluid and experiments are conducted in a forced convection loop at mass flux range of 400–1600 kg/m2s. Arrays of microchannel consist 5 (five) parallel straight microchannels with Width, Depth and Length dimension of 220 μm, 250 μm and 10 mm respectively. Flow boiling performances including heat transfer coefficient (HTC), pressure drop, two-phase flow instabilities and critical heat flux (CHF) have been studied in both the Silicon plainwall (smooth inner surface) and Silicon Nanowire (silicon nanostructured inner surface) microchannels. High speed flow visualizations have been performed at up to 70,000 frames per s (fps) to understand the difference in boiling mechanisms between Plainwall and SiNW. SiNW performs significantly enhanced HTC (up to 400% improvement), reduces flow boiling instabilities and pressure drop (up to 70% reduction) compared to Plainwall microchannels. However, little/insignificant effect of nanostructured surface has been observed on CHF. In addition, a major difference in two-phase flow regime development has been observed between the SiNW and Plainwall microchannels during flow visualization. Specifically, SiNWs introduce explosive bubble nucleation, reduce intermittent flow regimes (slug/churn), improve rewetting, maintain thin liquid film and thus, improve system performances.

Experimental Study of Subcavity in Supersonic Cavity Flow

An experimental study is conducted at Mach number 1.71 on rectangular open cavity of length-to-depth ratio () 2 with subcavity at the front wall. Subcavity at the front wall has been established as a passive control device. In the current study it is found that subcavity at the front wall can also act as a passive resonator. The dominance of fluid-dynamic or fluid-resonant oscillations accordingly causes the subcavity at the front wall to act as a passive control device or as a passive resonator in supersonic cavity flow. In the current study the presence of the fluid-dynamic and fluid-resonant cavity oscillations is clearly brought out and the inherent differences between the two types of oscillations are also highlighted. Fluid-resonant cavity oscillations are not often observed experimentally in most of the experimental works related to supersonic cavity flow. A total of six cavity models with different subcavity lengths are investigated and are also designated by the ratio of subcavity length to main cavity length (, 0.10, 0.20, 0.25, 0.30, and 0.40). High-speed schlieren flow visualization and unsteady pressure measurements are employed to gain further insight into the flow physics. High-speed schlieren flow visualization clearly indicates the presence of six different types of waves and five different flow features that are associated with the cavity flow field. Statistical analysis techniques, namely, fast Fourier transform, spectrogram, correlation, and coherence, are employed for analyzing the unsteady pressure data. Fluid-dynamic-type cavity oscillations are dominant in , 0.10 and 0.20 cavities. Pressure oscillations inside the cavity are suppressed as is increased from 0.00 to 0.20. As is increased from 0.20 to 0.25, the type of cavity oscillations changes from fluid-dynamic to fluid-resonant. This change in the type of cavity oscillations results in increased pressure oscillations inside the cavity for , 0.30, and 0.40. Distinct cavity tones at certain frequencies in power spectrum plots are classified into fluid-dynamic mode and fluid-resonant mode. Modal frequencies of fluid-resonant modes differ widely from those predicted by modified Rossiter formula. Correlation analysis suggests the presence of fluid-resonant behavior in , 0.30, and 0.40 cavities. With reference to cavity, there is maximum reduction of 30.2 dB in sound pressure level for the second fluid-dynamic mode, and there is also 4.4 dB reduction in overall sound pressure level for the cavity.

Experimental Characterization of Intrinsic Properties Associated With Air-Assisted Liquid Jet and Liquid Sheet

The present study focuses on experimental characterization of interfacial instability pertinent to liquid jet and liquid sheet in the first wind-induced zone. To accomplish this objective, the interfacial wave growth rate, critical wave number, and breakup frequency associated with air-assisted atomizer systems were extracted by utilizing high-speed flow visualization techniques. For a range of liquid to gas velocities tested, nondimensionalization with appropriate variables generates the corresponding correlation functions. These functions enable to make an effective comparison between interfacial wave developments for liquid jet and sheet configurations. It exhibits liquid sheets superiority over liquid jets in the breakup processes leading to efficient atomization.

Flow boiling of R245fa in a microgap with staggered circular cylindrical pin fins

In this study, flow boiling of refrigerant R245fa is investigated in a microgap of height 200 μm populated with a staggered pin fin array of diameter 150 μm and spacing 200 μm. For heat fluxes up to 498 W/cm2, mass flux values up to 7896 kg/m2 s, and inlet temperatures of 13 °C and 18 °C, average two-phase heat transfer coefficient up to 60 kW/m2 K are measured. High speed flow visualizations at frame rate of 2229 fps elucidate the flow boiling patterns inside the microgaps, including bubbly and foggy that are generated in the pin finned area. Surface temperatures are measured for heat fluxes up to 498 W/cm2 which enable determination of heat transfer characteristics.

Conserved spatio-temporal patterns of suction-feeding flows across aquatic vertebrates: a comparative flow visualization study.

Suction feeding is a widespread prey capture strategy among aquatic vertebrates. It is almost omnipresent across fishes, and has repeatedly evolved in other aquatic vertebrates. By rapidly expanding the mouth cavity, suction feeders generate a fluid flow outside of their mouth, drawing prey inside. Fish and other suction-feeding organisms display remarkable trophic diversity, echoed in the diversity of their skull and mouth morphologies. Yet, it is unclear how variable suction flows are across species, and whether variation in suction flows supports trophic diversity. Using a high-speed flow visualization technique, we characterized the spatio-temporal patterns in the flow fields produced during feeding in 14 species of aquatic suction feeders. We found that suction-feeding hydrodynamics are highly conserved across species. Suction flows affected only a limited volume of ∼1 gape diameter away from the mouth, and peaked around the timing of maximal mouth opening. The magnitude of flow speed increased with increasing mouth diameter and, to a lesser extent, with decreasing time to peak gape opening. Other morphological, kinematic and behavioral variables played a minor role in shaping suction-feeding dynamics. We conclude that the trophic diversity within fishes, and likely other aquatic vertebrates, is not supported by a diversity of mechanisms that modify the characteristics of suction flow. Rather, we suggest that suction feeding supports such trophic diversity owing to the general lack of strong trade-offs with other mechanisms that contribute to prey capture.

Effects of heat flux, mass flux and two-phase inlet quality on flow boiling in a vertical superhydrophilic microchannel

Saturated flow boiling experiments were conducted to investigate the influence of surface wettability on the hydraulic and thermal transport performance in a large width-to-height aspect ratio, one-sided heated rectangular microchannel with deionized water as the working fluid. The contact angles of the bare silicon wafer surface and superhydrophilic surface after deposited by a thin film of 100-nm-thickness silicon dioxide were 65° ± 3° and less than 5° respectively, both of which were utilized as heated surfaces of the microchannel. Parametric experimental studies were carried out with the inlet vapor quality varied from 0.03 to 0.1 and the wall heat fluxes spanned from 4 W/cm2 to 20 W/cm2, at various mass fluxes ranging from 120 to 360 kg/m2 s. High speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux, mass flux and two phase inlet quality on the local heat transfer coefficient, averaged heat transfer performance, two phase flow structure and pressure drop characteristics for surfaces with distinct surface wettability characteristics in the microchannel. Experimental results showed that the local heat transfer coefficient decreased first until approaching a minimum value and then increased towards the exit along the flow direction. Severe heat transfer deterioration was obtained for the bared silicon wafer surface with increased inlet vapor quality and heat flux, resulting from the local dryout phenomenon as can be verified by the flow visualization. While the heat transfer performance of the superhydrophilic surface was relatively constant due to continuous and uniform distribution of the thin liquid film on the heated surface during annular flow dominance and subsequent delay to partial dryout occurrence, which outperformed the untreated hydrophilic surface without additional pressure drop penalty.

Two-dimensional heat transfer coefficients with simultaneous flow visualisations during two-phase flow boiling in a PDMS microchannel

Infrared (IR) thermography was combined with simultaneous high speed flow visualisation and pressure measurements from integrated pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient (HTC) maps across the full domain of a polydimethylsiloxane (PDMS) high aspect ratio (a = 22) microchannel (Dh = 192 μm). High spatial and temporal resolution two-dimensional wall temperature measurements and pressure data were obtained for a range of mass fluxes (G = 7.37–298 kg m−2 s−1) and heat fluxes (q = 13.64–179.2 kW m−2) using FC-72 as the working liquid. The 3D plots of HTC provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow regime. The optical images from the channel top revealed the local flow regimes and were correlated with simultaneous thermal images obtained from the channel base. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with vapour-liquid dynamics and liquid film thinning (from the contrast of the optical images) which caused suspected dryout. The correlation between the synchronised at the same frame rate high-resolution thermal and optical images will assist in a better understanding of the heat transfer mechanisms during two-phase flow boiling in microchannels. This work intends to give a better insight into heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during a cycle of liquid–vapour alternations.

Multiscale modal analysis of an oscillating impinging gas jet

Oscillating impinging gas jets are encountered in many flow configurations, including fluidic devices, jet flow discharges into cavities and jet impinging onto deformable interfaces. For an incompressible flow, the self-sustained mechanism is referred to as hydrodynamic feedback and it is linked to the interaction between the flow field and its geometric confinement. This work presents an experimental characterization of the flow structures responsible for the hydrodynamic feedback, considering both a steady confinement and a pulsing confinement. The selected configuration consists of a planar gas jet impinging on an asymmetric interface consisting of a flat wall on one side of the stagnation region and bidimensional bump on the other. This bump is fixed in the steady condition and pulses in the unsteady conditions. Jet oscillation and interface tracking are carried out by means of high-speed flow visualization and image processing, combined with a time-frequency analysis in the Fourier and Wavelet spaces. The flow field is characterized via Time-Resolved Particle Image Velocimetry (TR-PIV) and analyzed with a novel modal decomposition, referred to as multi-scale Proper Orthogonal Decomposition (mPOD). This decomposition uses multiresolution analysis (MRA) on the correlation matrix to produce a set of PODs at multiple scales.Two physical mechanisms of the hydrodynamic feedback have been captured at largely different scales and the dynamic of their characteristic flow structures is presented. The first, referred to as fluidic mode, consists of a self-sustained oscillation of the impinging jet; the second, referred to as coupled mode, consists of a periodic downward deflection of the impinging jet, at the frequency of the interface pulsation.

Visualization of R134a flow boiling in micro-channels to establish a novel bubbly-slug flow transition criterion

In this study, the fundamental flow pattern characteristics of R134a flow boiling in a micro-channel heat sink were investigated based on high-speed flow visualization. Visual experiments were operated with R134a as the working fluid. The test section was composed with 20 parallel rectangular channels 500 μm wide, 500 μm deep, and 60 mm long. Mass flux and heat flux were variable from 164 to 573 kg/m2 s and 1 kW/m2 to 200 kW/m2, respectively, at the inlet temperature of 22 °C. Flow images were recorded with speed up to 10,000 frames/s. Bubbly flow, slug flow, churn flow, liquid lump flow, and annular flow were observed experimentally and the results plotted onto an x-G flow pattern map. For “M”-shaped heat transfer coefficient curve, five observed flow patterns were combined into one curve to reflect the heat transfer characteristics of flow boiling in micro-channels. Mechanisms of transition regions (bubbly-slug, slug-churn, churn-liquid lump, and liquid lump-annular) were established accordingly. The Active Nucleation Site Density Na was found to be the most significant parameter in regards to the bubbly-slug and slug-churn transition mechanisms. To verify the effect of the Active Nucleation Site Density (Na) on the bubbly-slug transition, the relations of Na and Boiling number at bubbly and slug flow regions were compared; the resulting slope features made the relations of Na and Boiling number easily distinguishable. Classification between the bubbly flow and slug flow of R134a was proposed based on these distinctions. The proposed criterion classified the transition region between the bubbly and slug flow. The proposed transition criterion was proven effective and feasible by comparison against previous transition criteria and datasets.

Overcharge and thermal destructive testing of lithium metal oxide and lithium metal phosphate batteries incorporating optical diagnostics

Lithium batteries have a tendency to fail violently under adverse conditions leading to the rapid venting of gas. Overcharge, thermal heating, and a combination of the two conditions are applied here to investigate the gas venting process. A test chamber has been constructed with data recordings including chamber pressure and temperature, battery voltage, current, and surface temperature as functions of time throughout the charging and failure processes. High-speed imaging and schlieren flow visualization are used to visualize the gas venting process. A direct comparison between lithium iron phosphate based K2 26650 and lithium nickel manganese cobalt oxide LG 18650 cells is made through a test series of the three failure methods. Failure under thermal, overcharge, and thermal-overcharge conditions are generally similar in terms of the gas venting process, but are observed to have increasingly energetic failures. The thermal-overcharge abuse condition demonstrates an ability to reconnect via internal short circuit even after an initial electrical failure seen as the refusal to accept charge. This reconnection is associated with a secondary, more energetic failure which can produce weak shock pressure waves.

Thermal characterization and optimization of pulsating heat pipes operating in a circulation mode

In the present study, thermodynamic characterization and thermal optimization of pulsating heat pipes (PHPs) operating in a circulation mode are conducted experimentally and analytically. A series of experiments is performed to identify the thermodynamic state of the vapor plugs inside the PHPs. Two-turn closed-loop PHPs are made of glass capillary tubes with inner diameters of 1.2, 1.7, and 2.2mm. Temperature, internal pressure, and high-speed flow visualization data are obtained for PHPs at various input powers in a vertical orientation inside which the flow in the steady state exhibits a circulating motion with a sporadic pulsating motion. It is experimentally found that when a PHP is operating in a circulation mode, the vapor plugs in the PHP exist in a saturation state and the saturation pressure of each vapor plug is nearly identical. This physically means the phase changes can be treated to occur at a constant saturation temperature corresponding to the system pressure along the entire tube. On the basis of the experimental findings, a one-dimensional numerical model is developed to predict the thermal performance of PHPs operating in a circulation mode. The numerical results are shown to be in close agreement with the experimental data within an error of 7%. Finally, thermal optimization is conducted using the proposed model. The number of turns (Nturn) is optimized under the given three-dimensional space (200×190×6 mm3) for various lengths of the condenser section (20, 40, and 60mm). According to the numerical results, there is an optimum number of turns at which the thermal performance is maximized in the case of PHPs with condenser length of 20mm, while the thermal performance of PHPs with condenser length of 40 and 60mm is monotonically enhanced as the number of turns increases. The optimum thermal performance results from the combined effect of the mass flux of circulating flow and the condensation area.