Superheated Layer Research Articles

An expansion effect based pseudo-boiling critical point model for supercritical CO<sub>2</sub>

Heat transfer deterioration (HTD) is one of the important issues in the study of supercritical fluid (SCF) heat transfer. However, when the SCF crosses the pseudo-critical point, the none-quilibrium process occurs in liquid, so SCF is very complicated. Recently, the existence of SCF pseudo-boiling on a macro scale has sparked controversy. There is still no unified understanding of the mechanism of gas-like and liquid-like transition affecting heat transfer. In this work, it is assumed that SCF has a macroscopic phenomenon similar to subcritical flow boiling. By analogy with subcritical boiling heat transfer, a boiling critical point model is proposed to describe the HTD in supercritical CO<sub>2</sub>. Our study reveals that the HTD caused by pseudo-boiling only occurs under large temperature gradient, which makes the superheated liquid-like layer cover the wall, and the gas-like and liquid-like may present different distribution forms, thus changing the heat transfer characteristics. When the wall temperature is higher than the pseudo-critical temperature and the enthalpy of the fluid layer covering the wall exceeds a certain value, the HTD may occur. The proposed theoretical model can explain the experimental results well, and the prediction accuracy of heat transfer correlation considering pseudo-boiling is greatly improved. In this work, the connection between supercritical heat transfer and subcritical heat transfer is established theoretically, which provides a new idea for studying the deterioration of SCF heat transfer, thus enriching the theory of supercritical heat transfer.

A modified force balance model for predicting bubble departure diameter and lift-off diameter in subcooled flow boiling

The calculation of heat flux for each component in the wall heat flux distribution model involves a crucial parameter, namely the characteristic bubble diameter. The bubble departure diameter and lift-off diameter are two of the most significant characteristic bubble diameters. In this paper, the bubble growth rate and bubble force analysis in subcooled flow boiling under various inclination angles of the heating surface are investigated, and the bubble departure and lift-off diameters are calculated based on force balance. According to the energy balance method, a superimposed bubble growth model is proposed that incorporates the effects of superheated layer evaporation around the bubble, microlayer evaporation, and condensation at the top of the bubble on the bubble growth rate. This paper proposes a force balance model for heating surfaces with varying inclination angles, based on accurate calculations of wall superheat. The model has been validated by a large amount of experimental data and exhibits superior accuracy and stability compared to existing models.

Bubble growth and departure behavior in subcooled flow boiling regime

Understanding the detailed dynamics of bubble ebullition cycle is central to effectively exploit coolant phase change in subcooled flow boiling. In the present study, the bubble growth rate and the bubble departure mechanism in subcooled flow boiling conditions are investigated. The bubble growth rate is estimated by employing an energy balance model, which ensures that the applied wall heat flux contribution to components, such as (i) microlayer evaporation, (ii) conduction through superheated layer region, and (iii) condensation heat transfer, are accounted. The bubble departure diameter and the type of departure (i.e., sliding or lift-off) are predicted using a simple force balance model on the vapor bubble. The implemented models are thoroughly validated against the benchmark experimental data. Furthermore, the influence of operating conditions, viz., pressure (1–3 bar), mass flux (200–1000 kgm−2s−1), heat flux (200–500 kWm−2), and the degree of subcooling (20–30 K) on the bubble dynamics, is investigated for subcooled flow boiling conditions. Based on the parameters considered, a flow regime map is developed to identify the bubble departure type and diameter for a given set of operating conditions. It was noticed that the bubble departure diameter was maximum at low pressure (1 bar), low mass flux (200 kgm−2s−1), high heat flux (500 kWm−2), and low degree of subcooling (20 °C). For all the values of pressure and degree of subcooling, the sliding mode of departure was noticed at low and high mass flux values, whereas bubble departs by lift-off for moderate values of mass flux.

Experimental investigation to quantify the influence of single artificial indentations on vapour bubble dynamics and the associated heat transfer rates

Dependence of single vapour bubble dynamics and the associated heat transfer phenomena on the type of artificial indentations created on the heated substrate has been studied. Experiments under nucleate boiling regime have been carried out for conical indentations of varying cone angles (2γ = 30, 60 and 90o) and cylindrical cavity keeping the depth of features constant (hc = 500 μm). Rainbow schlieren deflectometry has been employed for simultaneous mapping of bubble dynamics and thermal field in a completely non-intrusive and real time manner. Direct comparison of the rainbow schlieren images shows a relatively large partition of thermal energy through natural convection for conical indentation of small cone angle and cylindrical cavity. For large-angled conical indentation, increased availability of thermal energy for bubble growth process results in the formation of relatively larger-sized vapour bubble at a relatively slower rate. In addition, due to the relative differences in the amount of vapour entrapped inside the cavities between any two bubbling cycles, the consistency of bubbling features was found to be much higher for larger cone angled cavity. The evidence for increased partition of thermal energy transfer through natural convection has been seen in the form of an increase in the portion of superheat layer in the vicinity of vapour bubble for conical cavity with small cone angles from the whole-field temperature reconstructions. For ΔTsup = 10 K, the heat transfer performance of surfaces with conical cavity with large cone angle (2γ = 90o) is significantly much better than the other conical and cylindrical cavities.

Bubble growth models in saturated pool boiling of water on a smooth metallic surface: Assessment and a new recommendation

Prediction of bubble growth rate is very important for the development of accurate models for bubble departure diameter and thus the heat transfer rates in nucleate boiling. This paper presents an evaluation study to the existing homogeneous and heterogeneous bubble growth models using our experimental data for bubble growth in saturated pool boiling of deionized water on a plain copper surface. The experiments were conducted at pressures 1, 0.5 and 0.15 bar and superheat in the range 5.1 – 19.5 K. To start with, the paper presents a critical review on bubble growth models in homogeneous and heterogeneous boiling. It was found that homogeneous growth models achieved some partial agreement with the experimental data at some conditions and thus they should be used carefully in heterogeneous boiling. There was a good agreement between some of the models that were suggested based on the assumption that bubble growth occurs due to evaporation from the superheated boundary layer around the bubble. The models based on microlayer evaporation only could not explain the experimental data, i.e. partial agreement at some conditions. The model that predicted the data very well at all conditions was the “relaxation boundary layer” model by Van Stralen [25]. This model was generalized in the current study by suggesting two new empirical models for the departure diameter and departure time.

Пожарная опасность взрывных режимов испарения сжиженного природного газа

Introduction. The fire hazard of explosive regimes of liquefied natural gas (LNG) evaporation was analyzed on the basis of the published research findings. These regimes include the rollover and the rapid phase transition (RPT). Characteristics of explosive regimes of LNG evaporation. Rollover occurs in LNG storage tanks in case of spontaneous mixing of LNG layers having different temperatures and densities. These layers emerge when the “fresh” product is loaded into a vessel containing the residual amount of product stored there before. A rapid increase in the LNG evaporation rate accompanies a pressure rise inside the tank, which can exceed an allowable pressure of the tank. RPT occurs at a contact of LND and water in the case of a release of liquefied natural gas onto a water surface. An explosive evaporation of LNG can cause in this case a formation of a shock wave and a large-scale vapor cloud. Investigations of rollovers. It was mentioned that a stratification of LNG in a storage tank is a necessary condition of a rollover. Two layers with different temperatures and densities are formed during this stratification. A superheating of a lower layer occurs at a heat exchange between the LNG and tank walls with a decrease of a density of this layer. A preferential evaporation of light components of LNG (methane, nitrogen) takes place in the upper layer, and the density of this layer increases. When the densities of these layers are equalized a spontaneous mixing of these layers occurs with an explosive evaporation of the product in the lower superheated layer. A time delay of rollover can reach 60–70 hours after the supply of the “fresh” product into the tank with “old” product. Investigations of a rapid phase transition. An energy released at the explosive evaporation and a pressure in a shock wave depend on many factors such as a LNG release rate, a position of a source of the LNG release — over or under a water level, a product composition, water temperature etc. It was found that a pressure hazardous for buildings and structures can take place at distances 250–500 m from the point of the release. The empirical correlation was proposed connecting a water temperature at the RPT occurrence and the superheating tempe­rature of LNG at which a boiling takes place in the regime of a homogeneous nucleation. Conclusions. It was shown that a realization of the explosive regimes of the LNG evaporation increases a fire hazard of objects for a storage and a transportation of liquefied natural gas. Recommendations for a prevention of such regimes are formulated.

Experimental and LBM simulation study on the bubble dynamic behaviors in subcooled flow boiling

Bubble dynamic behavior is a fundamental issue in comprehending flow boiling process. In this paper, the bubble characteristics are studied in depth through experiment in conjunction with the Lattice Boltzmann method (LBM). Firstly, the visualized experiment in rectangular channels with double heating plates is performed during subcooled flow boiling. High-speed camera is employed to directly capture the macroscopic bubble dynamic behaviors under different working conditions. Furthermore, LBM model with a novel hybrid boundary scheme is developed to investigate mesoscopic bubble characteristics, including contact diameter, microlayer thickness profile and superheated layer thickness. The bubble diameter from LBM model is compared to experimental data with a mean relative error of 10.8%. The results of the current research find that the bubbles nucleating on the inside channel have smaller contact diameter ratio and larger inclined angle. Finally, combining macroscopic experiments and mesoscopic LBM studies, a model of the bubble growth rate is proposed for predicting bubble growth behavior. In the process of model development, three aspects for deciding the bubble growth rate are considered, including microlayer evaporation, heat diffusion from the superheated liquid layer, and condensation at bubble dome caused by subcooled flow boiling. The prediction model agrees well with the experimental results, with a mean relative error of 14.7%. The established bubble growth model will help to predict the heat transfer process during flow boiling.

A Comprehensive Model for Single Bubble Nucleate Flow Boiling

Abstract The single bubble dynamics and local thermal effects in a rectangular channel are investigated in this paper under subcooled nucleate flow boiling conditions. A combined effect of Marangoni and microlayer evaporation in flow boiling regimes has rarely been reported in open literature. Therefore, a comprehensive boiling model that combines the three submodels, namely, phase change model, microlayer evaporation model, and Marangoni model, is developed that accounts for small-scale physics such as evolution of superheat layer during bubble growth, microlayer evaporation, and scavenging of the superheated liquid during the bubble departure. The verification of model has been carried out through detailed flow and temperature field validation exercises of various bubble stages with recent experimental data reported in the literature. The effects of varying subcooled conditions and Reynolds number on bubble dynamics and the associated heat transfer rates have been examined. The study reveals a decreasing trend in the bubble diameter with increasing Reynolds number and degree of subcooling. It has also been observed that the bubble shape is affected by the Marangoni phenomena. Bubble shape slightly flattens during inception, gradually becomes spherical while sliding, and later elongates after liftoff. The individual contribution of microlayer heat flow (Qmicrolayer) is estimated to be around 22–40% for flow boiling conditions and it is the second-highest heat transfer contributor after the latent heat transfer. The results obtained from the proposed model show a good match with published data and indicate the significance of microlayer in the single bubble flow boiling heat transfer.

Simultaneous laser-induced fluorescence, particle image velocimetry and infrared thermography for the investigation of the flow and heat transfer characteristics of nucleating vapour bubbles

Boiling is an effective heat removal process, used for heat exchange and thermal management purposes in many technological applications, from the scale of microelectronic devices to nuclear reactors. However, the physical mechanisms involved in this process are not fully understood yet due to the complexity that arises from the many interacting underlying sub-processes involved in the nucleation, growth and detachment of bubbles that occur during the process. Here, we present an advanced methodology based on combined, synchronized high-speed infrared (IR) thermometry, ratiometric two-colour laser-induced fluorescence (2cLIF) and particle image velocimetry (PIV), along with sample results of an experimental investigation conducted in deionized water, aimed at elucidating the mechanisms involved in the bubble lifecycle. IR thermometry is used to measure the time-dependent 2-D temperature and heat flux distributions over a boiling surface, and 2cLIF is used to measure the time-dependent temperature-field in a vertical plane, in the liquid phase around developing bubbles. Furthermore, PIV is used to measure the velocity fields around the bubbles, in the same plane as 2cLIF. The investigation reveals and allows us to quantify fundamental heat transfer aspects such as the contribution of triple contact line evaporation to the bubble growth process, the dynamics of the near-wall superheated liquid layer, the mixing effect produced by bubble growth and departure, convection effects around the bubble, and quenching heat transfer. Specifically, we observe that, in our experiment, with slowly growing bubbles, the microlayer does not form, and the evaporation at the solid-liquid-vapour contact line contributes to approximately one third of the total heat transferred to the bubble. We also observed that the fluid that rewets the dry spot at the bubble base, as the bubble departs from the boiling surface, comes from the near-wall superheated thermal boundary layer adjacent to the bubble, i.e., it is warmer than the fluid in the bulk. We confirm this finding by modelling this quenching heat transfer phase as a transient conduction process.

Numerical simulation of single bubble evolution in low gravity with fluctuation

The nucleate boiling under low gravity was solved numerically, with the thermocapillary force considered in the momentum equations. The two-phase interface was captured with the phase field method, and the superheated layer was included in the model. The effects of thermocapillary convection and g-jitter (fluctuating gravity) on the bubble dynamics and heat transfer were explored. It was found that the thermocapillary convection shortened the growth period of the bubble by about 0.26 s in the low gravity (low-g). The affected area of the temperature and flow fields was wider with consideration of thermocapillary convection, and the heat transfer was enhanced consequently. Moreover, the bubble dynamics showed different characteristics in the conditions with and without the g-jitter. The velocity of the contact line fluctuated between positive and negative values in g-jitter. As the amplitude of the g-jitter increased from 0 to 0.04, the departure time and departure radius decreased by 31.2% and 14.1%, respectively. Nevertheless, the average heat flux increased with the increase of the g-jitter amplitude. The frequency of g-jitter showed a negative correlation with departure time and departure radius of the bubble, whereas average heat flux showed a positive correlation with the frequency of g-jitter.

Numerical Study of Nanosecond Pulsed Laser Impact on a Water Droplet

An integrated hydrodynamic model that involves a laser heat source, surface evaporation, and interfacial driving forces is developed to simulate the complex physical process of a nanosecond pulsed laser impacting on a water droplet. Droplet deformation tracked by the coupled level set and volume of fluid method is contrasted with the experimental results, and it is shown that the coupled model can be used to simulate the droplet expansion dynamics. From the simulation of droplet deformation, it can be found that, with increasing laser energy, the droplet becomes thinner. With the highest laser energy, the jet on the opposite side to laser impact is interpreted as the flow of high-speed fluid near the axis, which is also confirmed by the velocity field. The simulated temperature reaches or even exceeds the critical value, and the recoil pressure does not completely cover the whole hemispherical surface of the droplet. The liquid-removal depth, which represents the distance that the liquid-phase interface retreats during evaporation, is much shallower than the absorption depth. It can be concluded that propulsion is gained from the intense evaporation of the superheated layer.

Hybrid model of thin film boiling: Insights into the unique behavior and ultrahigh heat flux

Ultrahigh heat flux of over 1 kW/cm2 can be achieved by a new thin film boiling regime, of which the variation of boiling curve is considerably different from that of pool boiling. However, there is a lack of quantitative analysis for the distinctive thin film boiling. In this work, comprehensive comparison was made between the well-studied pool boiling and the newly-achieved thin film boiling. The boiling curve of thin film boiling was divided into three segments and mathematical models were set up for each segment in a way that correlations for pool boiling were adopted and modified according to the similarity and difference between pool boiling and thin film boiling, respectively. It was found that the modeling results agreed very well with the experimental results, indicating that the hybrid model revealed the underlying mechanism for the unique behavior and ultrahigh heat flux of the thin film boiling. In brief, the higher heat transfer coefficient or larger slope of the boiling curve was related to the special water supply mode of thin film boiling, in which the water flowed vertically through the heat surface and consequently enhanced heat transfer by promoting the spread of superheated layer and the bubbles on the heating surface. As for the ultrahigh heat flux and the unique negative slope of the boiling curve, it was attributed to the very thin yet continuously decreased thickness of the liquid layer, which was another essential feature of the thin film boiling. The hybrid model in this work can provide both quantitative insight for fundamental understanding and future guidance for practical application of thin film boiling.

New insights into physics of explosive water boiling derived from molecular dynamics simulations

In this paper the dynamics of explosive boiling of a 72 Å thick water film on the hot copper substrate with the plain surface is analysed. The analyses are based on the results of molecular dynamics (MD) simulations of the transient in which the thermostat temperature within the solid substrate is increased from 298 to 800K. The obtained results show that explosive boiling comprises several phases. The first phase involves rapid thermoacoustic pressure build-up in the near wall region due to intensive water heating and its volumetric expansion. The generated compression wave propagates through the liquid film reaching the peak value higher than critical water pressure. Due to reflection at the free film surface the original compression wave turns into an expansion wave. This event leads to a rapid pressure decrease and occurrence of tension stress. The second phase starts when the pressure enters the negative domain. This phase is accompanied with nucleation of vapour embryos and their subsequent growth in the superheated water layer in the near wall region. As the intensity of vapour phase generation is moderate, the liquid film in this phase is still exposed to tension. The stress recovery starts in the third phase when the water temperature in the near wall layer attains the spinodal value. This leads to a massive vaporization, nanobubble coalescence and thermal explosion, which causes spallation of the liquid film and lift-off of the liquid slug. The last phase is characterized by suppression of heat transfer by the expanding vapour layer. As a consequence, after reaching the maximum, the pressure decreases. This paper gives quantitative description and detailed insight into the mechanisms associated with each of the aforementioned phases.

Non-contact experiments to quantify the microlayer evaporation heat transfer coefficient during isolated nucleate boiling regime

Experiments to understand the relationship between dynamics of superheat layer, microlayer evaporation and bubble growth process during single bubble formation under saturated pool boiling regime have been conducted. Thin film interferometer and rainbow schlieren deflectometry have been employed in tandem to simultaneously map the transient evolution of microlayer and superheat layer along with the bubble growth process. During initial time instances of growth process, heat flux dissipated in the microlayer region is determined to be as high as ~1 MW/m2 for the range of experimental conditions employed. As a result, a large temperature drop is observed in the microlayer region. As the bubble growth process enters its final stages, reduced temperature levels of the heated substrate result into a significant reduction in the depletion rate of the inner core region of microlayer, while the evaporation rate of its peripheral region is seen to increase. The consolidated evaporative heat transfer coefficient for the entire bubble cycle is determined to be ~140 kW/m2K for q″ = 50 kW/m2 and 170 kW/m2K for q″ = 80 kW/m2. The corresponding accommodation factor required for predicting the evaporative heat transfer coefficient values through kinetic theory-based model is determined to be ~0.015.

Whole Field Measurements to Understand the Role of Varying Depths of Nucleation Site on Vapor Bubble Dynamics and Heat Transfer Rates

Abstract This work studies the possible effects of varying depths of cavity on bubbling features and the associated heat transfer rates in nucleate pool boiling regime. A single vapor bubble has been generated on a substrate with a cylindrical cavity at its center that acts as the nucleation site. Experiments have been conducted for three cavity depths (250, 500, and 1000 μm), while keeping its throat diameter constant at 200 μm. With the bulk fluid maintained under saturated conditions, for each cavity depth, surface superheat level has been varied in the range of ΔTsuperheat = 8, 10 and 12 °C. A gradient-based visualization technique, coupled with a high speed camera, has been employed to simultaneously map the changes in thermal gradients during the formation of the vapor bubble as well as bubble dynamic parameters. The image sequence obtained has been qualitatively and quantitatively analyzed to elucidate the dependence of bubbling features and various heat transfer processes on cavity depth. With an increase in the depth of cavity, the net effect of reduction in the available thermal energy due to the increased convection effects and significant depletion of superheated layer are identified as the dominant heat transfer processes that influence the bubbling features. Furthermore, based on the statistics of bubble departure characteristics, the cavity with higher depth (1000 μm) showed a much stable bubble formation with minimal variation in the bubble departure frequency as compared to the bubbling features from a cavity with smaller depth (250 μm). Evaporative heat transfer process has been identified as the primary cause for increased inconsistency of bubbling features at high superheat conditions for experiments performed for low cavity depths.

Sailing Droplets in Superheated Granular Layer.

We report instability of a superheated granular layer when a droplet is deposited on top of the layer. We find that the instability caused by evaporating vapor may trap or cause the droplet to sail away from the deposited position. The sailing motion is triggered by an unstable pressure distribution originated from fast fluidization of metallic grains. We provide a predictive model and experimental verification of the enabling conditions for sailing motion based on limiting criteria for fast fluidization.

Numerical study on heat and mass transfer behavior of pool boiling in LiBr/H2O absorption chiller generator considering different tube surfaces

Investigating the pool boiling process in the absorption chiller generator by studying the valid parameters may enhance the chiller?s COP. In the present study, the transient 2-D numerical modelling of LiBr/H2O solution pool boiling in the generator of the absorption chiller was carried out using the two-phase Eulerian-Eulerian approach, extended Rensselaer Polytechnic Institute boiling model and renormalization group k-? turbulence model. The numerical model was applied on three types of the bare, notched fin, and low fin tubes to investigate the effect of using fin on the boiling heat transfer rate in the generator of the absorption chiller and comparing it with the bare tube. Moreover, the numerical results were compared with the data obtained from the previous experimental studies to validate numerical modelling. A good agreement was achieved between numerical and experimental results. The results showed the evaporation mechanisms in the microlayer, evaporation in the three-phase (liquid-vapor-solid) contact line, and transient conduction the superheat layer for constant thermal heat flux and the three surfaces of the copper tube within a specific period from the boiling point of LiBr/H2O solution. The results also showed that the use of a notched fin-tube and low fin tube increases the non-homogeneous nucleation rate, causes the solution boil earlier than the bare tube, and reduces the required thermal energy in the generator of an absorption chiller.

Performance evaluation of alumina nanofluids and nanoparticles-deposited surface on nucleate pool boiling phenomena

Single bubble-based nucleate boiling phenomena of water and alumina-water nanofluids (0.005 and 0.01 vol% concentrations) have been studied under nucleate pool boiling regime for two different superheat levels. While nanofluids-based boiling experiments have been performed on plain substrate surface, experiments with water have been conducted on plain as well as nanoparticles-deposited surfaces. For developing an understanding of the influence of nanoparticles, simultaneous measurements of bubble dynamic parameters as well as associated thermal gradients field have been made using non-intrusive rainbow schlieren optical technique. A direct comparison of water and nanofluids experiments revealed significant changes in bubble dynamic parameters such as reduction in bubble departure diameter and growth time, increase in departure frequency and wait time for the case of nanofluids. Thermal gradients field showed more spreading out of the superheat layer adjacent to the heater substrate due to the addition of nanoparticles in the bulk liquid. In a subsequent study, surface modification of the heated substrate has been realized in the form of deposition of nanoparticles during nanofluid-based boiling experiments. The nanoparticles that get deposited on the surface during long time boiling experiments lead to surface modifications. The resultant nanoparticle-deposited surface is employed as the heater surface for conducting pool boiling experiments with water. Results of these experiments have been compared with those conducted using plain (smooth) heated substrate. Similar trends have been observed in the bubble dynamic parameters for the experiments with water on nano-deposited surface vis-a-vis nanofluids on plain surface. Heat transfer partitioning study revealed that natural convection component reduces and evaporative heat transfer increases significantly in the presence of nanoparticles. Plausible mechanisms for the observed trends have been investigated and discussed.

Influence of Pulsed Beams of Deuterium Ions and Deuterium Plasma on the Aluminum Alloy of Al–Mg–Li System

The damage and structural state of the surface layer of Al–Li–Mg samples composed of Al–5% Mg–2% Li (wt %) under pulsed action of power streams of high-temperature deuterium plasma and high-energy deuterium ions in the Plasma Focus (PF) device have been investigated. The radiation power density was q ~ 106 W/cm2; the pulse duration was 50–100 ns. Pulsed thermal heating and rapid cooling is established to lead to the melting and solidification of a thin surface layer of the alloy for several tens of nanoseconds. At the same time, in the superheated surface layer of the alloy, microcavities of a spherical shape are formed which is associated with intense evaporation of lithium into micropores within the heated layer. Thermal stresses caused by abrupt heating, melting, and cooling of a thin surface layer of metal result in formation of microcracks in the near-surface zone of the samples. The evaporation by the power electron beam of the elements of the anode material of the PF device (copper and tungsten) and their subsequent deposition onto the irradiated surface of the investigated samples in the form of droplets of submicron size are noted. It is shown that the thermal and radiation-stimulated processes generated in the alloy under the action of pulsed energy fluxes in the implemented irradiation regime lead to the redistribution of elements in the surface layer of the aluminum solution, contributing to an increase in magnesium content and the formation of magnesium oxide on the surface.

Performance of SiO2-water nanofluids for single bubble-based nucleate pool boiling heat transfer

Experiments on single bubble-based pool boiling have been performed under saturated bulk conditions to understand the influence of suspended nanoparticles on bubble dynamics and the associated temperature gradients field. Water and two concentrations of silica-water nanofluid (0.005 and 0.01% (V/V)) have been subjected to isolated nucleate pool boiling on a ITO-coated borofloat heater by supplying constant heat flux. Experimental measurements have been made in a purely non-intrusive manner by employing refractive index-based rainbow schlieren deflectometry technique. Bubble dynamic parameters, such as bubble departure diameter and departure frequency, wait and growth times etc. and the temperature gradients in the bulk fluid have been mapped simultaneously in the form of schlieren images. Results on bubble dynamic parameters revealed that these parameters show more variability and assume skewed normal distribution in the case of nanofluids whereas water experiments showed insignificant variations in these parameters. Of the significant changes, the bubble departure diameter reduced, departure frequency and growth rate increased with increasing concentration of nanofluids. Schlieren images corresponding to 0.005 and 0.01% (V/V) nanofluids showed more spreading out of the superheat layer adjacent to the heater substrate than that observed in the case of water experiments. Time-averaged values of natural convection and evaporative heat transfer rates associated with single bubble-based boiling indicated that both these rates decrease in the presence of dispersed silica nanoparticles. Reduction in the overall heat transfer rate in the case of nanofluids has been explained on the basis of direct experimental measurements. In this direction, the observed phenomenon has been attributed to the role played by the suspended nanoparticles in diffusing the thermal gradients (reflected in the form of the broadening of the superheat layer) adjacent to the heater substrate along with substantially increasing the wait time of bubble inception in the case of nanofluids as compared to water-based experiments conducted under same wall superheat conditions.