Side Heat Transfer Coefficient Research Articles

ZnO⿿propylene glycol⿿water nanofluids with improved properties for potential applications in renewable energy and thermal management

ZnO⿿propylene glycol⿿water (ZnO⿿PG⿿water) nanofluids of higher thermal conductivity and lower viscosity than PG⿿water mixture were prepared by the sequential method. The unsteady state heat transfer performance of ZnO⿿PG⿿water nanofluids revealed 16.5% increase in heat transfer rate for 2vol% nanofluids under constant heat flux conditions. The coolant side heat transfer coefficient increased by 29% on use of 2vol% ZnO⿿PG⿿water nanofluid for heat removal from constant temperature source (simulated thermal energy storage). The collection efficiency of solar collectors employing ZnO⿿PG⿿water nanofluid was found to increase linearly with nanoparticle concentration with about 9% increase in collection efficiency for 2vol% nanofluid. These results summarily indicate that ZnO⿿PG⿿water nanofluids perform better than PG⿿water mixture for solar energy collection and discharging of solar thermal energy storage.

Study of multi-twisted-tube gas cooler for CO2 heat pump water heaters

Multi-twisted-tube type heat exchangers applied in a transcritical CO2 heat pump as gas coolers were investigated. The influence of different numbers of inner tubes on the heat transfer and the pressure drop was discussed by means of experiments and theoretical analysis. The performances of two gas coolers with three and four inner tubes were measured and compared. It is found that heat transfer characteristics of the gas cooler with four inner tubes is better than that with three inner tubes. Based on the theoretical analysis, outlet temperature of water increases with increasing the number of inner tubes at the beginning and the increasing tendency becomes gentle when the number of inner tubes is more than four. But the pressure drop of gas cooler rises greatly with the increment of the number of inner tubes. It is implied from the analysis that the coefficient of performance of CO2 heat pump is not optimum when the outlet temperature of water becomes high by increasing mass flow rate of CO2 and decreasing the mass flow rate of water. In order to get both high COP and high outlet temperature of water, the water side heat transfer coefficient of the gas cooler needs improving.

Numerical investigation on conjugate heat transfer to supercritical CO2 in membrane helical coiled tube heat exchangers

ABSTRACTConjugate heat transfer to supercritical CO2 in membrane helical coiled tube heat exchangers has been numerically investigated in the present study. The purpose is to provide detailed information on the conjugate heat transfer behavior for a better understanding of the abnormal heat transfer mechanism of supercritical fluid. It could be concluded that the supercritical fluid mass flux and vertical/horizontal placement would significantly affect the abnormal heat transfer phenomenon in the tube side. The flow field of supercritical fluid is affected by both the buoyancy and centrifugal force in the conjugate heat transfer process. The local wall temperature and heat transfer coefficient in the tube side would rise and fall periodically for the horizontal heat exchanger, but this phenomenon will gradually disappear with the increase of the mass flow rate or fluid temperature in the tube side. The dual effects of buoyancy force and centrifugal force lead to the deflection of the second flow direction for the vertical placement, which further results in the heat transfer deterioration region on the top-generatrix wall for the downward flow being larger than that for the upward flow.

Heat transfer and flow resistance performance of shutter baffle heat exchanger with triangle tube layout in shell side

Effect of main structural parameters of shutter baffle heat exchanger with a triangle tube layout in the shell side on heat transfer and flow resistance performance is studied in the article. A periodic whole cross-sectional computation model is built for the heat exchanger in the numerical study. The effects of structural parameters are analyzed, including assembly mode of shutter baffles, shutter baffle pitch, strip inclination angles, and strip widths. The correctness and accuracy of numerical simulation method are confirmed with a laser Doppler velocimeter experiment. Based on the research results, correlations for heat transfer coefficient and pressure drop in shell side are presented. Using the field synergy principle, the heat transfer enhancement mechanisms of segmental baffle heat exchanger and shutter baffle heat exchanger in shell side are analyzed.

Corrosion and Heat Transfer Characteristics of Water Dispersed with Carboxylate Additives and Multi Walled Carbon Nano Tubes

This paper summarizes a recent work on anti-corrosive properties and enhanced heat transfer properties of carboxylated water based nanofluids. Water mixed with sebacic acid as carboxylate additive found to be resistant to corrosion and suitable for automotive environment. The carboxylated water is dispersed with very low mass concentration of carbon nano tubes at 0.025, 0.05 and 0.1 %. The stability of nanofluids in terms of zeta potential is found to be good with carboxylated water compared to normal water. The heat transfer performance of nanofluids is carried out on an air cooled heat exchanger similar to an automotive radiator with incoming air velocities across radiator at 5, 10 and 15 m/s. The flow Reynolds number of water is in the range of 2500–6000 indicating developing flow regime. The corrosion resistance of nanofluids is found to be good indicating its suitability to automotive environment. There is a slight increase in viscosity and marginal decrease in the specific heat of nanofluids with addition of carboxylate as well as CNTs. Significant improvement is observed in the thermal conductivity of nanofluids dispersed with CNTs. During heat transfer experimentation, the inside heat transfer coefficient and overall heat transfer coefficient has also improved markedly. It is also found that the velocity of air and flow rate of coolant plays an important role in enhancement of the heat transfer coefficient and overall heat transfer coefficient.

Working fluid charge reduction, part 1: CO2 fin-tube evaporator designed for light commercial appliances utilizing flow pattern based phenomenological prediction models

The present study focused on improving the thermal performance of CO2 evaporators while reducing their volume and thus significantly reducing the working fluid charge. To achieve this, the leading two-phase flow pattern and physically-based prediction models for flow boiling of CO2 and void fraction were selected from the literature, together with the leading air-side flow and heat transfer methods, and finally those for modelling oil effects on CO2 flow boiling. The effects of tube size, shape and wettability for significant performance improvements, miscible oil concentration, etc., were addressed while staying within the beverage industry's fabrication and operational limits. Compared to an actual existing design, the volume and refrigerant charge of the CO2 evaporator were reduced by 50% and 58.7%, respectively. The adverse influence of oil was found to reduce the mean CO2 side heat transfer coefficient and to increase the CO2 pressure drop by up to 11% and 94%.

Evaporator performance enhancement by pulsation width modulation (PWM)

In this work, the effects of pulsation width on heat transfer coefficient and pressure drop in an air-to-refrigerant evaporator is studied experimentally. The pulsation width can be controlled to a minimum period of 2 s. A general model is also developed to predict heat transfer performance and pressure drop. The overall heat transfer coefficient, refrigerant side heat transfer coefficient, and pressure drop are measured and compared to a baseline without pulsating flow. The results show that the overall and refrigerant-side heat transfer coefficients have improved about 27% and 123%, respectively, with pulsating flow. The refrigerant mass flux and pulsation period have an important effect on the heat transfer enhancement. The average pressure drop of the pulsating flow differs little from that of continuous flow, but the temporal pressure drop of pulsating flow is quite different and varies with mass flux and pulsation period.

CFD simulations of shell-side flow in a shell-and-tube type heat exchanger with and without baffles

Shell-and-tube heat exchanger has been extensively used in industrial and research fronts for more than a century. However, most of its design procedures are based on empirical correlations extracted from experimental data of long length shell and tube heat exchanger. In this paper, an attempt has been made to investigate the complex flow and temperature pattern in such a short shell and tube type heat exchanger, with and without baffles in the shell side. Heat exchangers of length by hydraulic diameter ratio between 7<L/Dh<21(0.15m<DS<0.6m) for unbaffled and L/Dh<7(DS=0.09m) for baffled heat exchangers are analysed using CFD code OpenFOAM-2.2.0 for different mass flow rates. It was observed that the cross flow near the nozzle region has a significant contribution towards the heat transfer, hence the conventional heat transfer correlations do not apply to these short heat exchangers. Furthermore, a sensitivity study of turbulence models was performed and it was observed that the standard k–ε model gives best results for the velocity profile as well as heat transfer, provided average y+ of the first node adjacent to the heat transfer surface is maintained greater than 15. The commonly used boundary conditions at the exit are not realistic, as it tends to give either incorrect flow and temperature fields, or the solution was found to diverge. Through a sensitivity study of the exit length, it was found that exit length to shell side velocity ratio of 2.5 is required for proper convergence. Finally the effect of flow field on shell side heat transfer coefficient and a comparison with analytical methods are presented.

One-dimensional numerical study of thermal performance of an organic Rankine cycle system using liquefied natural gas as a cold source for cold energy recovery

A hybrid liquefied natural gas vaporizer system using an organic Rankine cycle for cold energy recovery is proposed for a typical liquefied natural gas (LNG) receiving terminal. Seawater and LNG are used as the hot and cold sources, respectively. A one-dimensional numerical model is established to formulate the conjugated heat transfer and thermodynamic characteristics for the evaporator, condenser, thermolator, and other components of the system. A solution algorithm for the coupled problem is developed, and the temperature distribution profiles for the three working fluids, namely, seawater, propane, and LNG, are obtained. The effects of five typical operating parameters, namely, the inlet temperatures of seawater and LNG, the inlet mass flow rates of seawater and LNG, and the propane condensation pressure, on the system are investigated. In the evaporator, the major thermal resistance occurs in the tube side of propane, and the peak values for the inner side and total heat transfer coefficients are found in the two-phase zone. An inner-side peak heat transfer coefficient for LNG is located near the pseudo-critical temperature in the condenser. The heat transfer coefficients are distributed uniformly on both the inner and shell sides of the thermolator. The seawater inlet temperature and propane condensation pressure are the two most sensitive factors for thermal efficiency. The inlet LNG mass rate and inlet LNG temperature have opposite effects on the thermal efficiency and the organic Rankine cycle output power.

Heat Transfer Enhancement Due to Coolant Extraction on the Cold Side of Effusion Cooling Plates

Effusion cooling represents one of the most innovative techniques to limit and control the metal temperature of combustors liner, and recently, attention has been paid by the scientific community on the characterization and the definition of design practices of such devices. Most of these studies were focused on the heat transfer on the hot side of effusion cooling plates, while just few contributions deal with the effusion plates cold side convective cooling. This paper reports a numerical survey aimed at the characterization of the convective cooling at the effusion plates cold side. Several effusion holes spacing is accounted for in conjunction with representative operating conditions. The study led to the development of an empirical correlation for the prediction of the cold side heat transfer coefficient enhancement factor, EF: it expresses the EF related to each extraction hole as a function of the pressure ratio β and the effusion plate porosity factor σ.

Evaluation Of Inside Heat TransferCoefficient of Roll Bond Evaporator for RoomAir Conditioner

In all refrigeration and air conditioning equipment the evaporator is either a plate and fin evaporator, tube and fin or a plate and tubular evaporator. There were little changes in manufacturing methods and geometry. This paper presents a new design of evaporator, in which roll bond evaporator is used in room air conditioner. Presently the use of roll bond evaporator is limited to domestic refrigerators only. The study throws light on evaluating the inside heat transfer coefficient ( refrigerant side) of the roll bonded evaporator using R-22 as Refrigerant

Improvisation of Shell Side Heat Transfer Coefficient in Shell and Tube Heat Exchangers using Different Configurations – A Mini Review

Improvisation of Shell Side Heat Transfer Coefficient in Shell and Tube Heat Exchangers using Different Configurations – A Mini Review

Experimental investigation on performances of trisection helical baffled heat exchangers for oil/water–water heat transfer

The trisection helical baffled shell-and-tube heat exchangers have structural features of more suitable to the equilateral triangular tube layouts and less baffle parts. In particular the circumferential overlap trisection helical baffled shell-and-tube heat exchangers are of anti-shortcut structure that accommodates one row tubes in each circumferential overlapped zone between adjacent baffles for dampening shortcut leakage. The performance tests were conducted on both oil–water and water–water heat transfer in heat exchangers with equilateral triangle tube layout of 16 tubes including five helical baffle schemes with incline angles of 12°, 16°, 20°, 24°, 28° and a segmental baffled one for comparison. The test results show that both the shell side heat transfer coefficient ho and pressure drop Δpo increase but the comprehensive index ho/Δpo decreases with the increase of the mass flow rate of all schemes; and that the shell side heat transfer coefficient, pressure drop and the comprehensive index ho/Δpo decrease with the increase of the baffle incline angle at certain mass flow rate, except that the curves of comprehensive index ho/Δpo of 12° and 16° helical baffle schemes are almost coincide. The average values of shell side heat transfer coefficient, the comprehensive index ho/Δpo of the 12° helical baffled scheme are about 50% higher than those of the segmental one with almost same pressure drop. The correlation equations for shell side Nusselt number and axial Euler number are presented varying with axial Reynolds number, Prandtl number and helical baffle incline angle.

Improved transient heat transfer performance of ZnO–propylene glycol nanofluids for energy management

Experiments were performed to study the transient heat transfer characteristics of probe-ultrasonicated, surfactant-free, ZnO–propylene glycol (ZnO–PG) nanofluids under constant heat flux and constant bath temperature conditions. Under constant heat flux conditions, ZnO–PG nanofluids exhibited linear increase in heat removal rates with increasing nanoparticle concentration. An enhancement of 4.24% in heat transfer rate was observed with 2vol.% ZnO–PG nanofluid. The overall heat transfer coefficient and nanofluid-side heat transfer coefficient under constant bath temperature condition were found to increase with increasing nanoparticle volume concentration. About ∼26% increase in coolant side heat transfer coefficient was obtained with 2vol.% ZnO–PG nanofluid under constant bath temperature condition which could be attributed to thermal conductivity enhancement as well as improved natural convection due to viscosity reduction. Our results indicate that the surfactant-free, ZnO–PG nanofluids behave like liquid coolants but with improved thermal conductivity and specific heat and hence suitable for energy management.

Water Mass Flow and Air Relative Humidity on Performance of Air - Source Heat Pump Water Heater

The water mass flow and air relative humidity on the performance of air-source heat pump water heater was discussed in this paper. The results show that the energy efficiency of heat pump water heater is improved by correct the water mass flow and discharge pressure can be reduced, the value of the condenser temperature difference between the import and export of water also can be reduced. Increasing the air relative humidity, the more condensated water on evaporator surface effective, the greater the wetting area. In addition, the heat transfer of latent heat can be increased by the growth of air side and refrigerant side heat transfer coefficient.

Experimental Study on Oil to Water Heat Transfer Performance of Circumferential Overlap Trisection Helical Baffle Heat Exchangers

Experimental study on heat transfer performance of circumferential overlap trisection helical baffle heat exchangers of tilt angles 12°, 16°, 20°, 24°and 28° compared with a segmental baffle heat exchanger are conducted. The testing heat exchangers are constructed of a common cylinder case and replaceable tube bundle cores. The hot fluid is heat transfer oil with about 70 ℃ in the shell side and the cold fluid is cooling water with about 11 ℃ in the tube side. The results show that within the testing range, the overall heat transfer coefficient, the shell side heat transfer coefficient, pressure drop as well as comprehensive indexes ho/Δpo and ho/Δpo1/3 decrease with the increase of the tilt angle of the baffles, and the optimum scheme is the one with baffle tilt angle of 12°, and the ratios of the mean values of the shell side heat transfer coefficient ho, comprehensive index ho/Δpo and ho/Δpo1/3 of the helical scheme with baffle tilt angle of 12° over those of the segmental baffle scheme are 1.253, 1.391 and 1.285, respectively.

Development of experimental techniques for measurement of heat transfer rates in heat exchangers in oscillatory flows

Heat exchangers are important components of thermoacoustic devices. In oscillatory flow conditions, the flow and temperature fields around the heat exchangers can be quite complex, and may significantly affect heat transfer behaviour. As a result, one cannot directly apply the heat transfer correlations for steady flows to the design of heat exchangers for oscillatory flows. The fundamental knowledge of heat transfer in oscillatory flows, however, is still not well-established. The aim of the current work is to develop experimental apparatus and measurement techniques for the study of heat transfer in oscillatory flows. The heat transferred between two heat exchangers forming a couple was measured over a range of testing conditions. Three couples of finned-tube heat exchangers with different fin spacing were selected for the experiment. The main parameters considered were fin spacing, fin length, thermal penetration depth and gas displacement amplitude. Their effects on the heat exchanger performance were studied. The results were summarised and analysed in terms of heat transfer rate and dimensionless heat transfer coefficient: Colburn-j factor. In order to obtain the gas side heat transfer coefficient in oscillatory flows, the water side heat transfer coefficient is required. Thus, an experimental apparatus for unidirectional steady test was also developed and a calculation method to evaluate the heat transfer coefficient was demonstrated. The uncertainties associated with the measurement of heat transfer rate were also considered.

NUMERICAL ANALYSIS OF HEAT TRANSFER IN REFRIGERANT FLOW THROUGH A CONDENSER TUBE

In this thesis, heat transfer analysis of refrigerant flow in a condenser tube has been done. The main objective of this thesis is to find the length of the condenser tube for a pre-defined refrigerant inlet state such that the refrigerant at the tube outlet is saturated liquid or sub cooled liquid. The inlet refrigerant condition is saturated vapor. The problem involves refrigerant flowing inside a straight, horizontal copper tube over which air is in cross flow. Inlet condition of the both fluids and condenser tube detail except its length are specified. Here, changing pressure at discrete points along the tube is calculated by using two-phase frictional pressure drop and momentum equation mode. The heat transfer calculation has done by using condensation heat transfer correlations and simple heat transfer equations. The inside heat transfer coefficient calculated by using two phase heat transfer coefficient correlation. The unknown length of condenser tube has discrete many numbers of small elements. Each and every element has a calculations based on the pressure drop as well as heat transfer based on those correlation and every element calculated properties of refrigerant values has to check its states. At end of the iteration, a last element has reached saturated liquid condition of refrigerant and stops the entire calculation. So the length of condenser tube has been calculated by number of iteration and number of nodes with its distance. Predicted values were compared using another condensation heat transfer correlations. A computer-code using Turbo C has been developed for performing the entire calculation.

Effectiveness of evolutionary algorithms for optimization of heat exchangers

This paper comprehensively investigates performance of evolutionary algorithms for design optimization of shell and tube heat exchangers (STHX). Genetic algorithm (GA), firefly algorithm (FA), and cuckoo search (CS) method are implemented for finding the optimal values for seven key design variables of the STHX model. ∊-NTU method and Bell-Delaware procedure are used for thermal modeling of STHX and calculation of shell side heat transfer coefficient and pressure drop. The purpose of STHX optimization is to maximize its thermal efficiency. Obtained results for several simulation optimizations indicate that GA is unable to find permissible and optimal solutions in the majority of cases. In contrast, design variables found by FA and CS always lead to maximum STHX efficiency. Also computational requirements of CS method are significantly less than FA method. As per optimization results, maximum efficiency (83.8%) can be achieved using several design configurations. However, these designs are bearing different dollar costs. Also it is found that the behavior of the majority of decision variables remains consistent in different runs of the FA and CS optimization processes.

Assessment of GOTHIC and TRACE codes against selected PANDA experiments on a Passive Containment Condenser

Typical passive safety systems for ALWRs (Advanced Light Water Reactors) rely on the condensation of steam to remove the decay heat from the core or the containment. In the present paper the three-dimensional containment code GOTHIC and the one-dimensional system code TRACE are compared on the calculation of a variety of phenomena characterizing the response of a passive condenser submerged in a boiling pool. The investigation addresses the conditions of interest for the Passive Containment Cooling System (PCCS) proposed for the ESBWR (Economic Simplified Boiling Water Reactor).The analysis of selected separate effect tests carried out on a PCC (Passive Containment Condenser) unit in the PANDA large-scale thermal-hydraulic facility is presented to assess the code predictions. Both pure steam conditions (operating pressure of 3bar, 6bar and 9bar) and the effect on the condensation heat transfer of non-condensable gases heavier than steam (air) and lighter than steam (helium) are considered. The role of the secondary side (pool side) heat transfer on the condenser performance is examined too.In general, this study shows that both the GOTHIC and TRACE codes are able to reasonably predict the heat transfer capability of the PCC as well as the influence of non-condensable gas on the system. A slight underestimation of the condenser performance is obtained with both codes. For those tests where the experimental and simulated efficiencies agree better the possibility of compensating errors among different parts of the heat transfer models is outlined. Besides, the pool side heat transfer coefficient is indicated to differ between GOTHIC and TRACE, revealing that the model implemented in TRACE might be more accurate. Finally, both codes deviate more from the experiments for the tests with helium, which is explained by the emerging of a complex circulatory flow pattern in the experiments that the models fail to predict.