Ahmet Selim Dalkılıç

Measurement and Correlation of the Viscosity of Water-Based Al2O3 and TiO2 Nanofluids in High Temperatures and Comparisons with Literature Reports

In this article, the viscosity of two common nanofluids including Al2O3/water and TiO2/water is measured at high temperatures, and high concentrations of the nanofluids. The range of temperature is 15–60°C where the volume fraction of nanoparticles varies from 1 to 8%. Next, comparisons have been done with the most well-known theoretical and experimental reports in the literature. Finally, using the experimental data, a helpful correlation is presented.

Forced Convective Heat Transfer of Nanofluids - A Review of the Recent Literature

The forced convection of fluids has been investigated by numerous researchers, both experimentally and numerically. A good understanding of characteristics of nanofluid flowhas thoroughly been investigated in these studies. Since the nanoparticles behave more like a single-phase fluid than a solid–liquid mixture, it is assumed that nanofluids are ideally suited in the applications as their usage causes little or no penalty in pressure drop. In recent years, many researchers have tried to fill the gaps on this subject in the literature. To meet the demand for improving the performance of heat transfer equipment, re-examination of the individual components is considered to be essential. The addition of the nanoparticles to the base fluid is one of the significant issues for the optimal performance of heat transfer systems. This paper reports on most of the forced convective heat transfer literature occurring both in-tubes and in-channels regarding the use and preparation of nanofluids. The peer reviewed papers published in citation index journals up to 2012 have been selected for review in the paper. Classification of the papers has been performed according to the publication years. The critical information on the theoretical, experimental and numerical works is presented comprehensively for each paper.

A REVIEW ON THE HEAT-TRANSFER PERFORMANCE AND PRESSURE-DROP CHARACTERISTICS OF VARIOUS ENHANCED TUBES

The enhanced tube is a kind of the passive technique for improving the thermal performance of the heat exchangers with a little increase of the friction penalty. They have stated to use instead of the common smooth tubes for designing of the heat exchangers. The size of these heat exchangers can be reduced considerably by using the enhanced tubes instead of smooth tubes. Normally, they are divided into four groups: the corrugated tube, ribbed tube, grooved tube, and °uted tube. Compared with the common smooth tube, many researchers reported that use of the enhanced tubes dramatically increases the heat-transfer performance, both theoretically and experimentally. Focusing on the advantages of the enhanced tubes, this article summarizes the published studies on the heat-transfer and pressure-drop characteristics of the enhanced tubes, both experimental and quantitative investigations.

Nucleate Pool Boiling Heat Transfer Correlation for TiO2-Water Nanofluids

This paper is a continuation of the authors’ previous work on the nucleate pool boiling heat transfer of nanofluids [Suriyawong, A. and Wongwises, S., “Nucleate pool boiling heat transfer characteristics of TiO2-water nanofluids at very low concentrations,” Exp. Therm. Fluid Sci., Vol. 34, No. 8, 2010, pp. 992–999.] This study presents new correlation for predicting heat transfer coefficient for nucleate pool boiling of TiO2-water nanofluids at several low concentrations. Unlike most previous studies, the proposed correlation consists of various relevant factors. Two horizontal circular plates made from copper and aluminum with different surface roughness values are used as heating surfaces. Because the calculation concerns with properties of nanofluids, this research uses various correlations from previous studies to find the properties of nanofluids and the best one is selected for the presentation. Compared with measured data of nucleate pool boiling of water and nanofluids from present and previous studies, it was found that the developed correlation could be used for prediction at a certain level.

Experimental Research on the Similarity of Annular Flow Models and Correlations for the Condensation of R134a at High Mass Flux Inside Vertical and Horizontal Tubes

This paper presents an experimental investigation on the usage of annular flow models and correlations valid especially for horizontal tubes to the downward annular flow in the vertical test section. Condensation experiments are performed at the mass flux of 340 kg m−2 s−1 during co-current downward condensation of R134a in a vertical smooth copper tube having inner diameter of 8.1 mm and a length of 500 mm. The saturation temperatures are between 40–50°C, heat fluxes are between 12.8 and 45.36 kW m−2 , average qualities are ranging between 0.76–0.95. The experimental apparatus are designed to capable of changing the different operating parameters such as mass flow rate, condensation temperature of refrigerant, cooling water temperature and mass flow rate of cooling water etc and investigate their effect on heat transfer coefficients and pressure drops. Considering Chen et al.’s annular flow theory on the heat transfer coefficients that are independent from tube orientation as long as annular flow exists along the tube length, the average predicted condensation heat transfer coefficient of the refrigerant is determined by means of the annular flow model of Kosky and Staub, and Von Karman universal velocity distribution correlations using interfacial shear stress proposed for horizontal and vertical tubes separately. Some well-known annular flow correlations generally used for horizontal tubes in the literature were compared with experimental condensation heat transfer coefficient obtained from vertical tube data during annular flow conditions in the test section.Copyright

A Comparison of the Void Fraction Correlations of R134A During Condensation in Vertical Downward Laminar Flow in a Smooth and Microfin Tube

Void fractions are determined in vertical downward annular two-phase flow of R134a inside a 7 mm i.d. smooth and microfin tube. The experiments are done at average qualities ranging between 0.67–0.99. The mass fluxes are around 29 kg/m2 s. The pressures are between 0.77–0.9 MPa. The experimental setup is explained elaborately. Comparisons between the void fraction determined from 35 void fraction correlations are done. According to the use of various horizontal and vertical annular flow void fraction models together with the present experimental condensation heat transfer data, similar void fraction results were obtained mostly for the microfin and smooth tube. Effect of void fraction alteration on the momentum pressure drop is also presented.Copyright

Experimental Study on the Modeling of Condensation Heat Transfer Coefficients in High Mass Flux Region of Refrigerant HFC-134a Inside the Vertical Smooth Tube in Annular Flow Regime

This article presents an experimental investigation on the co-current downward condensation of R134a in a vertical smooth copper tube having inner diameter of 8.1 mm and a length of 500 mm. Condensation experiments are done at mass fluxes varying between 260 and 515 kg/m2-s. The condensing temperatures are 40°C and 50°C; heat fluxes are between 10.16 and 66.61 kW/m2. The quality of the refrigerant in the test section is calculated considering the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The average experimental heat transfer coefficient of the refrigerant is calculated by applying an energy balance based on the energy transferred from the test section. The average predicted heat transfer coefficient of the refrigerant is determined by means of the model of Kosky and Staub, and Von Karman universal velocity distribution correlations using different interfacial shear stress equations valid for annular flow in horizontal and vertical tubes to investigate the Chen et al. annular flow theory. The effects of heat flux, mass flux, and condensation temperature on the pressure drop are also discussed. A new correlation using a large number of data points for the turbulent condensation heat transfer coefficient is proposed.

Effect of hydrogen–diesel dual-fuel usage on performance, emissions and diesel combustion in diesel engines

Diesel engines are inevitable parts of our daily life and will be in the future. Expensive after-treatment technologies to fulfil normative legislations about the harmful tail-pipe emissions and fuel price increase in recent years created expectations from researchers for alternative fuel applications on diesel engines. This study investigates hydrogen as additive fuel in diesel engines. Hydrogen was introduced into intake manifold using gas injectors as additive fuel in gaseous form and also diesel fuel was injected into cylinder by diesel injector and used as igniter. Energy content of introduced hydrogen was set to 0%, 25% and 50% of total fuel energy, where the 0% references neat diesel operation without hydrogen injection. Test conditions were set to full load at 750, 900, 1100, 1400, 1750 and finally 2100 r/min engine speed. Variation in engine performance, emissions and combustion characteristics with hydrogen addition was investigated. Hydrogen introduction into the engine by 25% and 50% of total charge energy reveals significant decrease in smoke emissions while dramatic increase in nitrogen oxides. With increasing hydrogen content, a slight rise is observed in total unburned hydrocarbons although CO2 and CO gaseous emissions reduced considerably. Maximum in-cylinder gas pressure and rate of heat release peak values raised with hydrogen fraction.

Effect of the use of natural gas–diesel fuel mixture on performance, emissions, and combustion characteristics of a compression ignition engine

A compression ignition engine with a mechanical fuel system was converted into common rail fuel system by means of a self-developed electronic control unit. The engine was modified to be operated with mixtures of diesel and natural gas fuels in dual-fuel mode. Then, diesel fuel was injected into the cylinder while natural gas was injected into intake manifold with both injectors controlled with the electronic control unit. Energy content of the sprayed gas fuel was varied in the amounts of 0% (only diesel fuel), 15%, 40%, and 75% of total fuel’s energy content. All tests were carried out at constant engine speed of 1500 r/min at full load. In addition to the experiments, the engine was modeled with a one-dimensional commercial software. The experimental and numerical results were compared and found to be in reasonable agreement with each other. Both NOx and soot emissions were dropped with 15% and 40%, respectively, energy content rates in gas–fuel mixture compared to only diesel fuel. However, an increase was observed in carbon monoxide emissions with 15% natural gas fuel addition compared to only diesel fuel. Although smoke emission was reduced with natural gas fuel addition, there was a dramatic increase in NOx emissions with 75% natural gas fuel addition.

Experimental Study on Evaporative Heat Transfer and Pressure Drop of R-134a Flowing Downward Through Vertical Corrugated Tubes with Different Corrugation Pitches

This study reports an experimental investigation of evaporative heat transfer and pressure drop of R-134a flowing downward inside vertical corrugated tubes with different corrugation pitches. The double tube test section is 0.5 m long with refrigerant flowing in the inner tube and hot water flowing in the annulus. The inner tubes are comprised of one smooth tube and three corrugated tubes with different corrugation pitches of 6.35, 8.46, and 12.7 mm. The test runs are performed at evaporating temperatures of 10°C, 15°C, and 20°C; heat fluxes of 20, 25, and 30 kW/m2; and mass fluxes of 200, 300, and 400 kg/m2s. The experimental data obtained from the smooth tube are plotted with flow pattern map for vertical flow. Comparisons between smooth and corrugated tubes on the heat transfer and pressure drop are also discussed. It is observed that the heat transfer coefficient and frictional pressure drop obtained from the corrugated tubes are higher than those from the smooth tube. Furthermore, the heat transfer coefficient and frictional pressure drop increase as the corrugation pitch decreases. The maximum heat transfer enhancement factor and penalty factor are up to 1.22 and 4.0, respectively.