Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Performance of heat transfer and pressure drop in a spirally indented tube

In an effort to develop a heat transfer enhancement technique for low temperature applications such as utilization of LNG cold energy, an experiment was carried out to evaluate the heat transfer and the pressure drop performance for a spirally indented tube using ethylene-glycol and water solutions and pure water under horizontal single-phase conditions. The test tube diameter was 14.86 mm and the tube length was 5.38 m. Heat transfer coefficients and friction factors for both inner and outer surfaces of the test tube were calculated from measurements of temperatures, flowrates and pressure drops. Correlations of heat transfer coefficients in the spirally indented tube, which were applicable for laminar and turbulent regimes were proposed for inner, and outer surfaces. The correlations showed that heat transfer coefficients for the spirally indented tube were much higher than those for smooth tubes, increased by more than 8 times depending upon the Reynolds number. The correlations were compared with other correlations for various types of surface roughness. The effect of the Prandtl number on the heat transfer characteristics was discussed. The critical Reynolds number from the laminar flow to the turbulent flow inside the spirally indented tube was found to be around Re=1,000.     

Prediction of forced convective boiling heat transfer coefficient of pure refrigerants and binary refrigerant mixtures inside a horizontal tube

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect.     

Effect of particle ingestion on the fouling reduction and heat transfer enhancement of a No-Distributor-Fluidized heat exchanger

To overcome the fouling problem that is common in heat exchangers for waste heat recovery, a new type of fluidized heat exchanger was devised and tested. Fluidized bed heat exchangers are considered to be a good candidate for waste heat recovery flue gases due to their demonstrated ability to avoid fouling or to clean out deposition on heat transfer surfaces, but have a major drawback with significant pressure losses. These pressure drops typically associated with the distributor plate, which is a key component in constructing any conventional fluidized bed system, limit the applicability of fluidized bed heat exchangers for use as an energy saving device. In a new design, however, dilute gassolid particulate is maintained without having a distributor plate. The main feature of this no-distributor-fluidized (NDF) heat exchanger is the self-cleaning action by ingested circulating particles at minimal additional pressure loss. In the present study, a multi riser NDF heat exchanger of 7,000 kcal/hr capacity was built to evaluate its heat transfer performance and fouling reduction characteristics. To experimentally simulate the fouled condition, fuel rich combustion gas with soot was introduced to the heat exchanger, then a cleaning test was performed by introducing glass bead particles (600μm) inside the gas passage of the heat exchanger unit. Through the present experimental study, the performance degradation due to fouling was successfully demonstrated and the cleaning role of particle circulation was identified. It was also demonstrated that small amounts of circulating particles contribute not only to the fouling reduction on the gas side, but also to the heat transfer enhancement. Experimental operation data for 50 hours including accelerated fouling are obtained to simulate the long-term behavior of the system.     

Fouling effect of geothermal water scale on heat transfer around a tube in a bank

Seales deposited onto the heating surfaces of heat exchangers seriously reduce the heat transfer performance and also increase the hydrodynamic drag. Accordingly, fouling is an important problem in the design and operation of heat exchangers. Present paper investigates the heat transfer around in-line four circular cylinders on which geothermal water scales are uniformly distributed. The cylinders are settled in tandem with equal distance between neighbouring cylinders and only the test cylinder is heated under the condition of constant heat flux. It is found that the heat transfer of the in-line tube banks greatly varies with the fouling of geothermal water scale, especially its scale height. Further, the local and average Nusselt numbers strongly depend upon the cylinder spacing and the Reynolds number.     

Investigation of boiling heat transfer characteristics of two-phase closed thermosyphons with various internal grooves

The boiling heat transfer characteristics of two-phase closed thermosyphons with internal grooves are studied experimentally and a simple mathematical model is developed to predict the performance of such thermosyphons. The study focuses on the boiling heat transfer characteristics of a two-phase closed thermosyphons with copper tubes having 50, 60, 70, 80, 90 internal grooves. A two-phase closed thermosyphon with plain copper tube having the same inner and outer diameter as those of grooved tube is also tested for comparison. Methanol is used as working fluid. The effects of the number of grooves, the operating temperature, the heat flux are investigated experimentally. From these experimental results, a simple mathematical model is developed. In the present model, boiling of liquid pool in the evaporator is considered for the heat transfer mechanism of the thermosyphon. And also the effects of the number of grooves, the operating temperature, the heat flux are brought into consideration. A good agreement between the boiling heat transfer coefficient of the thermosyphon estimated from experimental results and the predictions from the present mathematical model is obtained. The experimental results show that the number of grooves and the amount of the working fluid are very important factors for the operation of thermosyphons. The two-phase closed thermosyphon with copper tubes having 60 internal grooves shows the best boiling heat transfer performance.     

Numerical simulation of fully developed flow and heat transfer characteristics in a curved tube with pulsating pressure gradient

Characteristics of fluid flow and convective heat transfer of a pulsating flow in a curved tube have been investigated numerically. The tube wall is assumed to be maintained at a uniform temperature peripherally in a fully developed pulsating flow region. The temperature and flow distributions over a cross-section of a curved tube with the associated velocity field need to be studied in detail. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. The time-dependent, elliptic governing equations are solved, employing finite volume technique. The periodic steady state results are obtained for various governing dimensionless parameters, such as Womersley number, pulsation amplitude ration, curvature ratio and Reynolds number. The numerical results indicate that the phase difference between the pressure gradient and averaged axial velocity increases gradually up to π/2 as Womersley number increases. However, this phase difference is almost independent of the amplitude ratio of pulsation. It is also found that the secondary flow patterns are strongly affected by the curvature ratio and Reynolds number. These, in turn, give a strong influence on the convective heat transfer from the pipe wall to the pulsating flow. The results obtained lead to a better understanding of the underlying physical process and also provide input that may be used to design the relevant system. The numerical approach is discussed in detail, and the aspects that must be included for an accurate simulation are discussed.     

折流板换热器壳程流体流动与传热特性数值模拟

根据流体动力学和计算传热学理论,建立了折流板管壳式换热器计算模型,运用CFD技术对换热器壳程流体的流动与传热问题进行了三维数值模拟,得到了不同壳程进口雷诺数Re条件下换热器壳程流体的流场和温度场。对数值模拟结果进行分析,以总传热系数h,壳程总压降Δp以及单位压力损失下的传热系数h/Δp作为换热器性能的衡量标准,分析了不同折流板间距和不同折流板圆缺高度时管壳式换热器壳程总传热系数h、总压降Δp以及h/Δp随壳程进口雷诺数的变化规律。结果表明:随着壳程进口流速的增大,换热器壳程总传热系数和总压降增大、h/Δp减小;在壳程流体流量不变的情况下,结合单位压力损失下的传热系数h/Δp,适当减小折流板间距或减小折流板圆缺高度,可提高换热器的换热性能。     

Flow and heat transfer measurements of film injectant from a row of holes with compound angle orientations

An experiment has been conducted on the flow and heat transfer characteristics of film coolant injected from a row of five holes with compound angle orientations of 35° inclination angle and 45° orientation angle. The Reynolds number number based on the mainstream velocity and injection hole diameter 3.58 x10⁴. Three-dimensional velocity, film cooling effectiveness and heat transfer coefficient data are presented at three different mass flux ratios of 0.5, 1.0 and 2.0. Flow entrainment has been found between the vortices generated by adjacent injectants. The injectant with compound angle orientation entrains not only the mainstream boundary layer flow but also the adjacent injectant. Because of the flow entrainment, the injectant. With compound angle orientation is characterized by a single vortex while two bound vortices are usually observed in the case of simple angle injection. The strength of the secondary flow depends strongly on the mass flux ratio, which shows significant influence on the film cooling effectiveness and heat transfer coefficient.     

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet (OPIJ) were experimentally investigated. The local heat transfer coefficients were measured using thermochromic liquid crystals. The jet mean velocity and turbulent intensity profiles were also measured along the plate. The jet Reynolds number (Re, based on the nozzle width) ranged from 10, 000 to 35,000, the nozzle-to-plate distance (H/B) from 2 to 16, and the oblique angle (α) from 60 to 90 degree. It has been found that the stagnation point shifted toward the minor flow region as the oblique angle decreased and the position of the stagnation point nearly coincided with that of the maximum turbulent intensity. It has also been observed that the local Nusselt numbers in the minor flow region were larger than those in the major flow region for the same distance along the plate mainly due to the higher levels in turbulent intensity caused by more active mixing of the jet flow.     

Condensation heat transfer correlation for smooth tubes in annular flow regime

Condensation heat transfer coefficients in a 7.92 mm inside diameter copper smooth tube were obtained experimentally for R22, R134a, and R410A. Working conditions were in the range of 30–40°C condensation temperature, 95–410 kg/m²s mass flux, and 0.15–0.85 vapor quality. The experimental data were compared with the eight existing correlations for an annular flow regime. Based on the heat-momentum analogy, a condensation heat transfer coefficients correlation for the annular flow regime was developed. The Breber et al. flow regime map was used to discern flow pattern and the Muller-Steinhagen & Heck pressure drop correlation was used for the term of the proposed correlation. The proposed correlation provided the best predicted performance compared to the eight existing correlations and its root mean square deviation was less than 8.7%.     

Ion slip effect on the flow due to a rotating disk with heat transfer

The steady hydromagnetic flow due to a rotating disk is studied with heat transfer considering the ion slip. The governing equations are solved numerically using finite differences. The results show that the inclusion of the ion slip has important effects on the velocity distribution as well as the heat transfer. On leave from: Department of Engineering Math. and physics, Fac. of Engineering, El-Fayoum University, El-Fayoum, Egypt.     

Characteristics of cooling water fouling in a heat exchange system

This study investigated the efficiency of the physical water treatment method in preventing and controlling fouling accumulation on heat transfer surfaces in a laboratory heat exchange system with tap and artificial water. To investigate the fouling characteristics, an experimental test facility with a plate type heat exchange system was newly built, where cooling and hot water moved in opposite directions forming a counter-flow heat exchanger. The obtained fouling resistances were used to analyze the effects of the physical water treatment on fouling mitigation. Furthermore, the surface tension and pH values of water were also measured. This study compared the fouling characteristics of cooling water in the heat exchange system with and without the mitigation methods for various inlet velocities. In the presence of the electrode devices with a velocity of 0.5m/s, the fouling resistance was reduced by 79% compared to that in the absence of electrode devices.     

Experimental studies on heat transfer in the annuli with corrugated inner tubes

Experimental heat transfer data for single-phase water flow in the annuli with corrugated inner tubes are presented. In the annuli with parallel flow, ten different annular arrangements are considered. For water (low rate in l,700<Re<13,000 regime, data for Nusselt numbers are presented. The results show significant effects of both the pitch to trough height ratio (P/e) and the radius ratio (r ^*). AsP/e becomes closer to 8 in the range below the radius ratio(r ^*) of 0.5. Nusselt numbers increase. However, Nusselt numbers decrease in the range above the radius ratio (r^*) of 0.5 because flow reattachment position becomes farther in the narrower clearance.     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

Effects of axial temperature gradient on momentum and heat transfer with oscillating pressure and flow

Effects of axial temperature gradient on heat transfer, momentum transfer and energy conversion mechanisms within a closed cylinder-piston apparatus are analyzed. Assuming that the gas density change is small, the first-order and steady second-order solutions of continuity, momentum and energy equations are obtained. The solutions show that there exists a steady circulating flow and the magnitude of the steady axial velocity increases as the axial temperature gradient increases. There exists not only an oscillating component of heat flux between the gas and the wall, but also a steady component whose direction depends on axial temperature gradient. It is shown that heat is pumped from the wall near the piston to the wall near closed-end for negative axial temperature gradient. Heat transfer relation for both oscillating pressure and oscillating flow conditions is proposed.     

A naphthalene sublimation study on heat/mass transfer for flow over a flat plate

It is important to completely understand heat/mass transfer from a flat plate because it is a basic element of heat/mass transfer. In the present study, local heat/mass transfer coefficient is obtained for two flow conditions to investigate the effect of boundary layer using the naphthalene sublimation technique. Obtained local heat/mass transfer coefficient is converted to dimensionless parameters such as Sherwood number, Stanton number and Colburnj-factor. These also are compared with correlations of laminar and turbulent heat/mass transfer from a flat plate. According to experimental results, local Sherwood number and local Stanton number are in much better agreement with the correlation of turbulent region rather than laminar region, which means analogy between heat/mass transfer and momentum transfer is more suitable for turbulent boundary layer. But average Sherwood number and average Colburnj-factor representing analogy between heat/mass transfer and momentum transfer are consistent with the correlation of laminar boundary layer as well as turbulent boundary layer.     

Development of a device for the measurement of thermal and fluid flow properties of heat exchanger materials

As yet, no standard equipment exists for the measurement of heat transfer through porous materials, such as metal foams (metals with a high volume fraction of porosity). Most research in this area has been carried out using bespoke test rigs. Here the creation of a test rig specifically developed for the measurement of the heat transfer of metal foams is reported. This method has been applied to laboratory made samples processed by replication and examples of commercially available aluminium foams (Duocel and Corevo), and should be suitable for the testing of all materials with comparable permeability. As this equipment is new and unique, the design will be discussed in detail, along with the various tests that were performed to ensure reliability and consistency with other methods and published data.     

A study on the improvement of heat transfer performance in low temperature closed Thermosyphon

The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having 50, 60, 70, 80, 90 internal grooves. Three different working fluids (distilled water, methanol, ethanol) are used with various volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube. The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum inclination angle for this study is 20°-30° from the horizontal position.     

Hall effect on unsteady couette flow with heat transfer under exponential decaying pressure gradient

The unsteady Couette flow of an electrically conducting, viscous, incompressible fluid bounded by two parallel non-conducting porous plates is studied with heat transfer taking the Hall effect into consideration. An external uniform magnetic field and a uniform suction and injection are applied perpendicular to the plates while the fluid motion is subjected to an exponential decaying pressure gradient. The two plates are kept at different but constant temperatures while the Joule and viscous dissipations are included in the energy equation. The effect of the ion slip and the uniform suction and injection on both the velocity and temperature distributions is examined.