Convective Flow Boiling Heat Transfer Research Articles

Convective Flow Boiling Heat Transfer Enhancement with Aqueous Al2O3 and TiO2 Nanofluids: Experimental Investigation

Convective Flow Boiling Heat Transfer Enhancement with Aqueous Al2O3 and TiO2 Nanofluids: Experimental Investigation

Convective flow boiling heat transfer in an annular space: N-heptane/water case in a bubbly sub-cooled flow

The subcooled flow boiling heat transfer characteristics of n-heptane and water is conducted for an upward flow inside the vertical annulus with an inner gap of 30 mm, in different heat fluxes up to 132kW.m-2, subcooling max.:30C, flow rate: 1.5 to 3.5lit.min-1 under the atmospheric pressure. The measured data indicate that the subcooled flow boiling heat transfer coefficient significantly increases with increasing liquid flow rate and heat flux and slightly decreases with decreasing the subcooling level. Although results demonstrate that subcooling is the most effective operation parameter on onset of nucleate boiling such that with decreasing the subcooling level, the inception heat flux significantly decreases. Besides, recorded results from the visualization of flow show that the mean diameter of the bubbles departing from the heating surface decreases slightly with increasing the flow rate and slightly decreases with decreasing the subcooling level. Meanwhile, comparisons of the present heat transfer data for n-heptane and water in the same annulus and with some existing correlations are investigated. Results of comparisons reveal an excellent agreement between experimental data and those of calculated by Chen Type model and Gungor–Winterton predicting correlation.

Experimental investigation on thermo-physical properties and subcooled flow boiling performance of Al2O3/water nanofluids in a horizontal tube

In the present experimental work, forced convective flow boiling heat transfer under subcooled region was analysed with Al2O3/water nanofluid flow in a horizontal tube. Nanofluids at various wt.% such as 0.01%, 0.05%, 0.1%, 0.5% and 1% were prepared using two-step method. Stability of nanofluids was measured using Zeta potential analysis. Thermo-physical properties such as the thermal conductivity and viscosity of all test samples were estimated over 25°C-65 °C. An experimental test rig was setup to study the convective heat transfer using nanofluids. The effects of concentration, heat flux, flow rate, surface roughness on heat transfer coefficient were measured. The experimental result showed good stability in nanofluids without any surfactants. A significant increase in convective heat transfer coefficient (HTC) was found due to enhancement in the thermo-physical properties of nanofluids. Increase in pressure drop was registered with the rise of concentration and flow rate. Finally, performance evaluation criterion (PEC) showed that Al2O3 nanofluids can increase the overall thermal performance of the system by 94% in comparison with pure water.

Reforming of methanol with steam in a micro-reactor with Cu–SiO2 porous catalyst

In the present work, we report the results of a series of experiments for the hydrogen production via steam reforming of methanol with Cu–SiO2 porous catalyst coated on the internal walls of a micro-reactor with parallel micro-passages. The catalyst was prepared by coating copper and silica nanoparticles on the internal surface of the microchannel via convective flow boiling heat transfer, followed by a calcination procedure at 973 K and therefore, the catalyst does not require any supportive material, which in turn reduced the complexity and cost of the preparation. The experiments were conducted at reactant flow rates of 0.1–0.9 lit/min, operating temperatures of 523–673 K, catalyst loading of 0.25 gr to 1.25 gr and at heat flux value of 500 kW/m2. Results of the experiments showed that the methanol conversion can reach 97% at catalyst loading of 1.25 gr. It was also found that with an increase in the gas hourly space velocity (GHSV) of the reactants, the methanol conversion decreases, which was attributed to the decrease in the residence time, the suppression in diffusion of reactants into the pores of the catalyst, and also the decrease in the average film temperature of the reactor. The highest methanol conversion was obtained at gas hourly space velocity of 24,000 ml/(gr.hr) and T = 773 K and for molar ratio of methanol to water of 0.1. The molar ratio of methanol to water also influenced the thermal response of the reactor such that the surface temperature profile of the micro-reactor was more decreased at low methanol/water molar ratios.

Can the heat transfer coefficients for single-phase flow and for convective flow boiling be equivalent?

During the past few decades, heat transfer during convective flow boiling inside pipes has been widely studied with the goal of unveiling the physics of the process. Different heat transfer mechanisms have been suggested based on different assumptions. This fact has resulted in a large number of models including different dimensionless numbers and in some cases up to a dozen of adjusted parameters. Here, we show that the convective flow boiling heat transfer coefficient is equivalent to the one for single-phase flow when the influence of the vapour velocity is taken into account.

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m−2, mass fluxes range of 353–1,059 kg m−2 s−1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.

Convective boiling and particulate fouling of stabilized CuO-ethylene glycol nanofluids inside the annular heat exchanger

Thermal performance of convective flow boiling heat transfer and particulate fouling of CuO/EG nanofluids has been experimentally studied inside the annular heat exchanger. CuO nanoparticles were well-dispersed and stabilized using a new combinational method (adding surfactant, stirring, pH control and sonication) in ethylene glycol (EG) as the base fluid in different weight fractions of nanoparticles (0.1–0.4%). Despite stabilizing the nanofluids, a considerable boiling heat transfer reduction due to the fouling resistance has been reported. Subsequently, scale formation and particulate fouling of nanofluids in term of fouling resistance has experimentally been investigated. Influences of operating parameters on the fouling resistance and heat transfer coefficient are investigated and briefly discussed.

Application of thermodynamic models to estimating the convective flow boiling heat transfer coefficient of mixtures

A large number of experiments has been performed to measure the forced convective and nucleate flow boiling heat transfer coefficient of three different none-volatile mixtures at different heat fluxes (up to 175kWm−2) and five different volumetric concentrations (10–50% of heavier component). The test mixtures include water/glycerol, water/monoethylene glycol (MEG), and water/diethylene glycol (DEG). The experimental apparatus provides conditions to investigate the influence of the main operating parameters such as: heat flux, concentration, and flow rate of fluid on the forced convective and flow boiling heat transfer coefficient. It is shown that physical properties of the mixtures have a considerable effect on the prediction of flow boiling heat transfer coefficients by the predictive correlations. In almost all of the predictive correlations, physical properties are strongly involved which can be estimated by different thermodynamic models. This work demonstrates that thermodynamic models for the calculation of specific heat, liquid density and heat of vaporization do not obtain identical results and consequently, the heat transfer coefficient obtained from a specified predictive correlation (Chen type model) can be tolerated according to the used thermodynamic model for the calculation of the physical properties. This point has been ignored by the investigators and they compare their experimental data with the correlations without specifying that, which one of the thermodynamic models has to be used for the obtaining of the thermo-physical properties. After reading the present study, a new vision can be opened to the readers interested in prediction of the flow boiling heat transfer coefficient and may help the researchers to reliably predict the thermo-physical properties of fluids particularly for forced convective and boiling phenomena.

Using micro-structural surface features to enhance the convective flow boiling heat transfer of R-134a on aluminum

The convective flow boiling performance of R-134a on various micro-structured aluminum surfaces produced using advanced manufacturing techniques is examined. More specifically, the boiling heat transfer coefficient of R-134a on a bare aluminum surface and three micro-structurally enhanced surfaces is calculated and compared to currently available, commercially enhanced tubes. Two of these enhanced surfaces were produced using photolithography and reactive ion etching techniques, and the third surface was produced by means of laser-ablation. Experiments were performed for mass fluxes between 100 and 600kg/m2s and for heat fluxes between 5 and 30kW/m2. Excellent agreement with published correlations was observed during initial benchmarking on the baseline surface. The boiling heat transfer coefficients on Surfaces 2 and 3 (produced by reactive ion etching) were only modestly higher than the baseline surface; whereas, the heat transfer coefficients on Surface 4 were as much as 90–100% higher than the baseline surface which was attributed to the porous layer of sintered aluminum particles formed on top of the surface during the laser ablation process. This increase in boiling heat transfer performance was then further enhanced through the application of a hydrophobic coating to the surface. The laser ablation technique presented in this paper has broad applicability to metals other than aluminum, and its ability to produce complex micro-structural surface geometries is of special interest in advanced flow boiling and high heat flux applications where current heat transfer surface designs are often costly to fabricate.