Inlet Subcooling Temperature Research Articles

Experimental investigation of heat transfer and pressure drop in copper manifold microchannel heat sinks

Over the past decade, various two-phase cooling solutions have been implemented to dissipate power in high heat flux electronic components. The manifold microchannel heat sink is an efficient thermal management solution that reduces heat resistance while also limiting the pumping power. However, there is limited work available on small form factor manifold microchannels with traditional heat sinking materials like copper. In this study, an experimental investigation is conducted to evaluate the heat transfer and pressure drop performance of copper manifold microchannel heat sinks using an environmentally friendly refrigerant, R1233zd(E), as the working fluid. A microchannel plate and a manifold plate, both made of oxygen-free copper, are sandwiched together to form the manifold microchannel test sample assembly. To compare the effects of geometric parameters, a total of six manifold microchannel test samples are investigated, including two types of manifold plates and three types of microchannel plates. The experiments are performed with refrigerant mass flow rates ranging from 5 to 15 g/s and inlet subcooling temperatures ranging from 3 to 10 K. The heat transfer coefficient increases with a decreasing number of manifolds and with an increase in the number and width of the microchannels. The pressure drop decreases in configurations with more microchannels, more manifolds, and larger microchannel widths. Compared to traditional microchannels, manifold microchannels significantly reduce the total pressure drop. However, their heat transfer performance at higher heat flux conditions is limited by the trapped bubbles phenomena. In comparison to traditional microchannels, manifold microchannels demonstrate much higher performance in dissipating the same amount of heat flux while limiting pumping power, as indicated by the COP.

Experimental study on flow boiling of HFE-7100 in rectangular parallel microchannel

With the rapid development of microelectronic technology, the integration and power of chip are increasing. Heat dissipation with high heat flux in limited space has become a bottleneck restricting the efficient and stable operation of the microelectronic devices. Flow boiling in microchannel heat sink is one of the most essential candidates for solving this problem. It has been shown that remarkable high heat transfer performance can be achieved through the liquid-to-vapor change process, which can dissipate a large amount of heat from a small area. In addition, dielectric fluorinated fluids, such as HFE-7100, HFE-7200, and FC-72, are especially suitable for cooling microelectronic devices, because of their excellent safety and environmental characteristics. However, dielectric fluorinated fluids have poorer thermophysical properties than water. Thus, the flow boiling heat transfer characteristics of dielectric fluorinated fluids can be different from those of water. In this work, flow boiling heat transfer and flow characteristics of HFE-7100 in a rectangular parallel microchannel are investigated. The tests are conducted at mass fluxes from 88.9 to 277.8 kg·m<sup>–2</sup>·s<sup>–1</sup>, inlet subcooling temperature from 20.5 to 35.5 ℃ and effective heat flux from 12 to 279 kW·m<sup>–2</sup> at nearly atmospheric pressure. The effects of mass flux, inlet subcooling temperature, effective heat flux and vapor quality are examined and analyzed. Additionally, flow visualization is also obtained to explain the heat transfer mechanism during the experiments. The results show that the boiling hysteresis is observed for HFE-7100 at low inlet subcooling temperature, and the increasing inlet subcooling temperature and mass flux can delay the onset of nucleate boiling. The increases of inlet subcooling temperature and mass flux can enhance the two-phase heat transfer coefficient. And the two-phase heat transfer coefficient is significantly dependent on the inlet subcooling temperature in the slug flow, while it is significantly dependent on the mass flux in the annular flow. The two-phase pressure drop increases drastically as the effective heat flux increases. And the two-phase pressure drops with different mass fluxes at constant vapor quality are obviously different between the slug flow and the annular flow. Furthermore, the experimental data are compared with four predicted values of the literature. It is found that the correlation of Lockhart has the best statistical agreement with an MAE of 19.6% and over 85% of points in the deviation bandwidth of ±30%. The results in this paper give valuable theoretical guidance for designing and optimizing heat dissipation equipment for microelectronic devices. By utilizing HFE-7100 as the coolant and microchannel heat sinks in flow boiling, it is possible to enhance the stability and reliability of the electronic devices. Additionally, the heat transfer performance associated with different heat fluxes can be improved by regulating the inlet subcooling and mass flow rate. Finally, the two-phase pressure drop correlation proposed by Lockhart can be employed to predict the pump power for heat dissipation equipment.

Effect of RIBS/FINS and Aspect Ratio on Flow Boiling Characteristics in Conventional Channels

Abstract In this work, experiments are conducted with conventional rectangular channels of two different aspect ratios (AR = w/d) for the horizontal boiling flow conditions at atmospheric pressure. Distilled water was used as the working substance. The heat transfer coefficients (HTC) were measured for mass fluxes and heat fluxes ranging from 85.94 kg/m2-s to 343.77 kg/m2-s and 10 kW/m2 to 100 kW/m2, respectively, and at inlet subcooled temperatures of 303 K, 313 K, and 323 K. Visualization of the boiling phenomenon was done using a high-speed camera for the two channels under similar conditions. The results show that the AR has a dominant effect on the HTC. At low heat flux values, higher HTC was noticed for the channel of higher AR (AR = 1.25) whereas, at high heat flux conditions, the HTC is higher for the channel of lower AR (AR = 0.2). With an increase in inlet subcooled temperature, the HTC decreased for both channels due to increased thermal boundary layer thickness and reduced bubble formation. Further, the channel of AR = 1.25 with ribs/fins performed better than the smooth channel due to the high bubble nucleation rate.

Achieving ultra-uniform temperature distribution with a heterogeneous wettability surface for two-phase flow in microchannels

Microchannel-based two-phase schemes have been proposed as an efficient solution for thermal management. However, the non-uniform temperature distribution in flow boiling limits phase-change performance and impedes commercial applications. In this study, we introduce a novel surface featuring heterogeneous wettability, comprising upstream superhydrophobic, middle plain, and downstream superhydrophilic regions. Hierarchical nanowires are employed to achieve the desired wettability, which can promote bubble departure and enhance capillary pumping. This wettability distribution facilitates upstream bubble nucleation and downstream liquid supply, significantly improving temperature uniformity. Moreover, surface contact angle variations effectively contribute to flow behavior improvement by restricting vapor reversal and fostering churn flow. Our results demonstrate that heat aggregation in the middle region and temperature restriction in the downstream region cause the high-temperature region to migrate upstream. This temperature redistribution stimulates axial heat transfer, substantially suppressing inlet subcooling and restricting temperature gradients. Compared to the plain surface, the heterogeneous wettability surface afforded an 87.8% reduction in temperature difference between Tw1 and Tw5 at approximately 120 °C, suggesting an ultra-uniform temperature distribution. Additionally, the average heat transfer coefficient showed a 35.1% enhancement rate at a wall heat flux of 100 W/cm2, and the performance evaluation criterion showed a 44.2% enhancement rate at an effective heat flux of 190 W/cm2. These results indicate that the proposed surface can efficiently regulate temperature distribution, maintain performance stability, and enhance heat transfer. The heterogeneous wettability surface has considerable potential for practical applications in high-power component heat dissipation.

Thermal Performance Analysis of Micro Pin Fin Heat Sinks under Different Flow Conditions

Due to microscale effects, the segmented microchannels or micro pin fin heat sinks emerged as a high thermal management solution. In this context, the present work analyzes the influence of different heights of square micro pin fins with an aligned array and investigates their influence on pressure drop and heat transfer behavior. The HFE-7100 is used as the working fluid, and the pressure drop and surface temperature behavior are analyzed for different mass fluxes and inlet subcooling. The single-phase flow was analyzed numerically using the computational fluid dynamics (CFD) software ANSYS FLUENT® for comparing the simulation results with the experimental data, showing that the highest micro pin fins configuration provides a more uniform and lowest wall temperature distribution compared to the lowest configuration. There is a good agreement between the experimental results and the numerical analysis, with a mean absolute error of 6% for all the considered parameters. For the two-phase flow condition, experimental tests were performed, and for the highest subcooling, an increase in mass flux causes an enhancement in the heat transfer for low heat flux; by increasing heat flux, there is a gradual predominance of boiling heat transfer over convection as the heat transfer mechanism. The pressure drop drastically increases with the vapor amount flowing into the system, regardless of the pin fin height; the boiling curves for the higher fin height show a much smaller slope and a smaller wall superheat than the fin with the smallest height, and consequently, a high heat transfer performance. A larger region of the heat sink is filled with vapor for lower inlet subcooling temperatures, degrading the heat transfer performance compared to higher inlet subcooling temperatures.

Experimental investigation on flow boiling of HFE-7100 in a microchannel with pin fin array

An experimental investigation on flow boiling heat transfer and pressure drop characteristics in a microchannel with an array of in-line circular pin fins was carried out. The tests with HFE-7100 as a working fluid were conducted at mass fluxes from 189 to 374 kg/m2∙s, inlet subcooling temperatures from 20.4 to 34.4 °C and effective heat fluxes from 17.4 to 239.3 kW/m2 near atmospheric pressure. The effects of mass flux, inlet subcooling temperature, heat flux and vapor quality were examined and analyzed. Flow visualization was also obtained during the experiments to explain the heat transfer mechanism. The boiling hysteresis was observed, which may be attributed to the low surface tension of HFE-7100. The experimental data were compared with various published data of dielectric fluorinated fluids, and many well-known correlations of pressure drop and two-phase heat transfer coefficient for pin–fin microchannels. The improved correlations of pressure drop and two-phase heat transfer were proposed, which demonstrated good predictions for both experimental data of the present and previous studies.

Experimental investigation on quenching behavior during reflooding in a 3 × 3 dual-cooled annular rod bundle

The internally and externally cooled annular fuel is an innovative fuel geometry proposal for advanced PWR, which could provide a substantial increase of power density while maintaining or improving safety margins. The quenching behavior of annular fuel during reflood phase in LOCA is more complicated than cylindrical solid fuel, owing to the geometrical difference and the coupling effect of dual-cooling. However, the investigation focusing on quenching characteristics in annular fuel geometry is very little. In the present study, an experimental study on quenching behavior of bottom reflood in a 3 × 3 dual-cooled annular rod bundle was carried out under various test conditions (reflood velocities from 0.025 to 0.15 m·s−1, inlet subcooling from 10 to 80°C, peak cladding temperature from 300 to 900°C, and linear power density from 0 to 1.58 kW·m−1). Self-designed indirect-heating annular rods were employed to provide an axial uniform power distribution in this experiment. It was found that the quenching process on external and internal surfaces of annular rod bundle was almost synchronous. The parametric effects on quenching behavior, i.e. precursory cooling rate, quench front propagation velocity (Vqf) and minimum film boiling temperature (Tmin), were investigated in detail. Earlier occurrence of quenching downstream the grid spacer was also observed under low inlet subcooling and high peak cladding temperature conditions. In addition, several correlations for Tmin were assessed against the present results. Kim correlation was in good agreement with the present data, with the mean absolute errors of 7.81% and 7.55% for outer and inner channels respectively.

Bubble Lift-Off Diameter and Frequency in Ferrofluid Subcooled Flow Boiling

Due to the importance of the boiling phenomena and the need of studying the bubble nucleation in different conditions, a series of experiments have been performed in the present study to investigate a magnetic nanofluid subcooled flow boiling phenomenon inside an annulus. The test section is made of outer glass and inner stainless-steel tubes. The effect of three important parameters, including mass flux, inlet subcooling temperature, and pressure on the bubble liftoff diameter and frequency in the absence and presence of a magnetic field caused by quadrupole magnets, have been considered. The experiments were visualized by using a superzoom camera to study the bubble behaviors on the heating surface. The obtained results show that increasing the mass flux, subcooling temperature, and pressure contributes to decreasing bubble liftoff diameter both in the absence and presence of the magnetic field during the flow boiling. The bubble liftoff diameter decreases by applying the magnetic field, and the bubbles lift from the surface faster. Moreover, the bubble liftoff frequency showed stochastic behavior such as dependence on heat transfer surface, bubble nucleation site properties, and fluid velocity. The obtained results show that the bubble liftoff frequency decrease with increasing the fluid mass flux.

Theoretical Analysis of Type-I Flow Instability of Natural Circulation

Fukuda and Kobori proposed three types of Leginegg instabilities and five types of density wave instabilities, according to different mechanisms of density wave instabilities. The instability dominated by gravity pressure drop is classified as Type-I flow instability. The instability dominated by friction pressure drop is classified as Type-II flow instability. And the instability dominated by accelerated pressure drop is classified as Type-III flow instability.In present work, the Type-I flow instability between rectangular parallel channels of natural circulation was studied theoretically. Models including two-phase flow instability, natural circulation system components were established in combination based on the homogenous model. A computational program was written. The program was validated with experiments, and the results matched well with the experiment data. The marginal stability boundary (MSB) maps under different parameters were obtained by using nondimensional numbers Nsub and Npch. The influence of different kinds of pressure drop, inlet subcooling temperature of heating channels of type-I flow instability were analyzed separately. And the phenomena were explained from mechanism and the change of void fraction. The results show that the induce of inlet subcooling temperature enhances the stability of system. With the increase in system pressure, the system stability of natural circulation is enhanced.

Subcooled Flow Boiling Heat Flux Enhancement Using High Porosity Sintered Fiber

Passive methods to increase the heat flux on the subcooled flow boiling are extremely needed on modern cooling systems. Many methods, including treated surfaces and extended surfaces, have been investigated. Experimental research to enhance the subcooled flow boiling using high sintered fiber attached to the surface was conducted. One bare surface (0 mm) and four porous thickness (0.2, 0.5, 1.0, 2.0 mm) were compared under three different mass fluxes (200, 400, and 600 kg·m−2·s−1) and three different inlet subcooling temperature (70, 50, 30). Deionized water under atmospheric pressure was used as the working fluid. The results confirmed that the porous body can enhance the heat flux and reduce the wall superheat temperature. However, higher porous thickness presented a reduction in the heat flux in comparison with the bare surface. Bubble formation and pattern flow were recorded using a high-speed camera. The bubble size and formation are generally smaller at higher inlet subcooling temperatures. The enhancement in the heat flux and the reduction on the wall superheat is attributed to the increment on the nucleation sites, the increment on the heating surface area, water supply ability through the porous body, and the vapor trap ability.

Enhancement of Subcooled Flow Boiling Heat Transfer with High Porosity Sintered Fiber Metal

We conducted experimental research using high-porosity sintered fiber attached on the surface, as a passive method to increase the heat flux for subcooled flow boiling. Two different porous thicknesses (1 and 0.5 mm) and one bare surface (0 mm) were compared under three different inlet subcooling temperatures (30, 50 and 70 K) and low mass flux (150–600 kg·m−2·s−1) using deionized water as the working fluid under atmospheric pressure. The test section was a rectangular channel, and the hydraulic diameter was 10 mm. The results showed that the heat flux on porous surfaces with a thickness of 1 and 0.5 mm increased by 60% and 40%, respectively, compared to bare surfaces at ΔTsat = 40 K at a subcooled temperature of 50 K and mass flux of 300 kg·m−2·s−1. An abrupt increase in the wall superheat was avoided, and critical heat flux (CHF) was not reached on the porous surfaces. The flow pattern and bubble were recorded with a high-speed camera, and the bubble dynamics are discussed.

Two-phase interfacial area characteristics and transport under the transition from subcooled boiling to saturated boiling flows

This study focuses on the analysis of the transition from the subcooled boiling to saturated boiling flow. During this process, vapor generates drastically in a short developing length, which makes it difficult to predict the interfacial characteristics of two-phase flow. To investigate the characteristics of this process, experiments were performed in a vertical annulus with 19.1 mm inner diameter and 38.1 mm outer diameter. The two-phase interfacial parameters including time-averaged local vapor fraction, interfacial area concentration, and bubble interfacial velocity are obtained using high-temperature miniature four-sensor electrical conductivity probes. In this study, the conductivity probe signal is processed using the newly-developed signal processing algorithm developed for small spherical bubbles by Shen and Nakamura (2014) , instead of the algorithm used in the previous subcooled boiling experimental studies Ozar (2009) and Kumar et al. (2019), [3] for large bubbles developed by Kataoka et al. (1986). The flow conditions cover a wide range of inlet subcooling temperature at 12.9–21.2 °C, heat flux at 57.9–197.3 kW/m2, and inlet flow rate at 0.265–0.536 m/s. The vapor fractions are cross-validated with impedance void meter measurements. The state-of-art two-group interfacial area transport equations (IATE) [6], (Ozar, 2009) and (Brooks and Hibiki, 2016) is evaluated with the experimental data. The result shows that the two-group IATE gives fairly good predictions on the transition phenomena. Besides, a defect was identified in the condensation model (Park et al., 2007) used in IATE, which is the bubble boundary diameter classifying the two condensation mechanisms: heat-transfer-controlled and inertia-controlled condensation. A rigorous approach to estimate this boundary diameter is established in this study.

A modified wall boiling model considering sliding bubbles based on the RPI wall boiling model

The bubble sliding is an important phenomenon in subcooled boiling flow, which has a significant influence to heat transfer, while the RPI wall boiling model does not consider the influence of sliding bubbles directly. This paper proposes a modified wall boiling model considering the sliding bubbles, and the simulation results of the modified model are compared with the experimental data to verify the accuracy of the model. According to the results, the modified model is applicable for the mass flux from 648.4 to 1630 kg/(m2•s), with the heat flux range of 227 to 306 kW/m2, the inlet subcooled temperature range of 30 to 50 K and the pressure range of 0.2 to 0.4 MPa. In addition, bubble departure diameter, bubble lift-off diameter, bubble sliding distance and the distribution of heat flux are discussed in this paper.

Numerical simulation of flow instability in a circular tube under pulse heating condition

Energy systems based on inertial confinement fusion or fusion-fission hybrid reactor generate energy in the form of pulse heating, which may lead to new flow instability phenomenon in the cooling system. In order to make thermal hydraulic design of pulse heating energy system safe and reliable, it is necessary to deeply understand the oscillation characteristics of flow and heat transfer under the pulse heating condition. As for the first stage of this complicated research, the numerical simulation of coolant flow in a circular tube under pulse heating condition is executed in this paper. The oscillation characteristics of flow and heat transfer under different pulse heating conditions are discussed in detail, and the effect of heating power, pulse heating period and duty ratio are studied. Four flow development stages (stage A – stage D) are identified, and three kind of flow oscillation phenomenon are found according to the numerical results with increasing the pulse heating power. The analysis shows that oscillation is mainly caused by pulse heating itself or the combination of pulse heating and density wave oscillation (DWO). Increasing the pulse heating period is beneficial to decrease flow oscillation, and decreasing the duty ratio, the flow oscillation is more obvious. The flow development stage maps are also given in the plane of average power and inlet subcooled temperature. With the increase of period, the stage D appears more easily at lower average power. When the duty ratio decreases, the region of stage C in instability map becomes larger.

Simulation study on flow instability in parallel narrow rectangular channels for downward flow with different heating modes

The flow instability can lead to damage of equipment in nuclear reactors and especially the core of plate-fuel element under low pressure. It is critical to predicting the onset of flow instability (OFI). In this research, a simulation of flow instability in parallel narrow rectangular channels for downward flow under low pressure has been investigated by RELAP. The sensitive analysis of system parameters on heating power and outlet void fraction at OFI were studied and the two-dimensional flow instability boundaries map of two parallel narrow rectangular channels has been obtained. To be closer to reality, the different heating modes including the cosine heating mode in the channel and the asymmetry heating mode in the parallel system have been considered in calculations and compared with the routine results. It is found that the system with cosine heating mode and that with symmetric heating mode are more stable than with uniform heating mode and with asymmetric heating mode, respectively. The difference between the OFI of different heating modes increase as the inlet subcooled temperature increases.

Theoretical investigation of two-phase flow instability between parallel channels of natural circulation in rolling motion

In present work, the two-phase flow instability between rectangular parallel channels of natural circulation under static and rolling conditions was coupled studied theoretically. Models including two-phase flow instability, natural circulation system components, and the additional force were established in combination based on the homogenous model. A computational program was written in FORTRAN language which was solved by Gear multi-value method by using control volume integrating method. The program was validated with experiments, and the results matched well with the experiment data. The marginal stability boundary (MSB) maps under different parameters were obtained by using nondimensional numbers Nsub and Npch. The influence of different kinds of pressure drop, inlet subcooling temperature of heating channels, system pressure, valve resistance, venturi flowmeters resistance, structure height, rolling condition, and the interaction effect of natural circulation and two-phase flow instability between rectangular parallel channels were analyzed. The results show that with the increase in system pressure and venturi flowmeters resistance, the system stability of natural circulation is enhanced. The influence of inlet subcooling temperature is nonlinear. The increase in valve resistance leads to the instability of system. The increase in structure height does not change system stability significantly. The rise in rolling angle and period both reduce the system stability.

NUMERICAL SIMULATION OF SUBCOOLED BOILING INSIDE THE NARROW RECTANGULAR CHANNEL OF FUEL PLATE FOR DOWNWARD FLOW

In this research, considering the structural parameters and operating conditions for NP reactors, a numerical simulation of downward sub-cooled boiling flow in a vertical rectangular channel under atmospheric pressure was carried out by VOF model of the FLUENT. The Lee model of FLUENT was used to realize the phase transition process of water liquid and water vapor. And CSF (Continuum Surface Force) model was adopted to handle surface tension. The wall super-heat at the Onset of Boiling (ONB) was calculated under different conditions. The simulation results were consistent with experimental data. The impact of different parameters on ONB was analyzed which showed that the distance from the entrance to ONB obviously increases with increasing inlet velocity and subcooled temperature. The increase of heat flux improved the wall super-heat at ONB, but no significant effect of inlet subcooled temperature and inlet velocity was found. In addition, the heat transfer coefficient along the direction of flow was calculated and analyzed. The results showed that the heat transfer coefficient increased significantly with increasing heat flux. However, it was not a simple monotonic relationship between heat transfer coefficient and the inlet velociety or inlet temperature. Moreover, the ONB and the heat transfer coefficient of present work were compared with the case of single-heated.

Subcooled flow boiling on hydrophilic and super-hydrophilic surfaces in microchannel under different orientations

An experimental investigation of subcooled flow boiling in the rectangular microchannel with hydraulic diameter of 0.94 mm (5.01 mm × 0.52 mm) for different orientations on hydrophilic and super-hydrophilic surfaces is conducted utilizing deionized water as the working fluid. The contact angles of the bare silicon surface and the nano-silica surface prepared through the plasma enhanced chemical vapor deposition method are 65° ± 3° and less than 5° respectively. With an inlet subcooling temperature of 10 °C, the mass fluxes and heat fluxes are tested in the range of 200–500 kg/(m2 s) and 4–25 W/cm2 respectively, while the orientation angles are 0° (bottom-heated horizontal flow), 90° (vertical upflow), 180° (top-heated horizontal flow) and 270° (vertical downflow). Significant appearances at the onset of nucleate boiling (ONB) accompanied with bubble nucleation are observed in the boiling curves. An earlier occurrence of critical heat flux (CHF) is presented for vertical downflow. With the direction of buoyancy opposite to the fluid inflow, the buoyancy force impedes the departure and movement of bubbles against the inertia, causing easier bubble coalescence and elongation as well as prolonged dry-out, which deteriorates heat transfer. At low mass fluxes the pressure drop for the case of vertical downflow is the highest among others, whereas at high mass fluxes the bottom-heated horizontal flow exhibits the maximum pressure drop. Most intense pressure drop fluctuation is obtained for vertical downflow, indicating the most severe flow instability under this orientation. The effect of orientation is weakened with the increment of mass flux due to the dominance of inertia force. At the mass flux of 200 kg/(m2 s), the super-hydrophilic surface delays CHF without increased pressure drop penalty in the vertical orientation, while in the horizontal orientation the effect of surface wettability seems more significant.

Numerical simulation of vapor volume fraction in a vertical channel under low-pressure conditions

This numerical study involved investigating void behavior under low-pressure subcooled flow boiling by using an Eulerian approach (two-fluid model). In the simulation, a vertical pipe with a length of 0.15 m and diameter of 0.01229 m was considered. Different levels of uniform wall heat flux, mass flux, and inlet subcooling temperature were applied although a constant pressure of 1.65 bar was used for all the simulations. A sensitivity study of the empirical coefficients used to access the predictive capacity of the existing mass transfer models was conducted. Thus, the k-epsilon model was used for the turbulence of the fluid. The axial vapor volume fraction profile, liquid temperature is compared at the operating pressure. Furthermore, the most sensitive flow characteristics of the channel were identified. The results indicated that the predictions of numerical phase evolution relative to the experimental observations were in good qualitative agreement with those obtained in extant studies. Additionally, the changes in drag coefficients were helpful in precisely predicting the void fraction. A commercial CFD solver was used for the implementation of the model.

Experimental study on the Onset of Nucleate Boiling in a narrow rectangular channel under transversely non-uniform and uniform heating

Onset of Nucleate Boiling (ONB) in a narrow rectangular channel was experimentally studied under transversely non-uniform and uniform heating conditions. The experiment was performed under various mass fluxes and inlet subcooling conditions. The behavior of the bubbles was recorded using a high-speed camera, and was analyzed using an image processing technique. In the case of uniform heat flux, the wall temperature was uniformly distributed in the transverse direction. In the case of non-uniform heat flux, the wall temperature had the lowest value in the middle of the heated surface, and increased along the transverse direction toward the edges. At the same mass flux and inlet subcooling temperature, the thermal power at the ONB under non-uniform heat flux was lower than that under uniform heat flux. However, the local heat flux and wall temperature at the ONB were similar at both heating conditions. In the subcooled boiling region, it was found that the boiling heat transfer under non-uniform heat flux was lower than that under uniform heat flux. Additionally, the slope of the wall temperature-thermal power curve after the ONB was higher in the non-uniform heat flux case. Using the results elicited from image processing, the evaporation and quenching heat transfers were evaluated to analyze the differences in boiling heat transfers. The boiling heat transfer was less affected by evaporation and quenching in the case of non-uniform heat flux.