Experimental study of forced convective heat transfer in a different arranged corrugated channel

In this study, heat transfer rate for sinusoidal corrugated channel has been experimentally investigated. Three different type sharp corrugation peak fins and a plain surface were used in the experiment. Results were carried out for constant heat flux of 616 W/m², varied Reynolds number Re 1500 to 8000 for the corrugation angle (27, 50 and 22/60°) and channel height of 5 and 10 mm. Nusselt number (Nu), convection heat transfer coefficient (h), Colburn factor (j) and enhancement ratio (E) against Reynolds number (Re) have been studied. The effects of the wavy geometry and channel height have been discussed. The increase of corrugated angle gave rise to a heat transfer rate.     

Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes

This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m², mass fluxes of 200–700 kg/m² s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.     

Effect of corrugated plates in an in-phase arrangement on the heat transfer and flow developments

In the present study, the numerical results of the heat transfer and flow developments in the corrugated channel under constant heat flux conditions are presented. The test section is the channel with two opposite corrugated plates which all configuration peaks lie in an in-phase arrangement. The corrugated plates with three different corrugated tile angles of 20°, 40°, and 60° are tested with the height of the channel of 12.5 mm. The model was simulated for the Reynolds number and heat flux in the ranges of 400–1600 and 0.5–1.2 kW/m², respectively. The flow and heat transfer developments are simulated by using the k-ε standard turbulent model. A finite volume method with the structured uniform grid system is employed for solving the model. The predicted results are validated by comparing with the measured data. There is reasonable agreement from the comparison between the numerical data and experimental data. Effects of relevant parameters on the heat transfer and flow developments are discussed. Due to the breaking and destabilizing in the thermal boundary zone, the corrugated surface has significant effect on the enhancement of heat transfer and pressure drop.     

3-D Numerical study on the correlation between variable inclined fin angles and thermal behavior in plate fin-tube heat exchanger

In this study, the heat transfer enhancement and pressure drop values of seven different fin angles with plain fin-tube heat exchangers were investigated. The numerical simulation of the fin-tube heat exchanger was performed by using a three dimensional (3-D) numerical computation technique. Therefore, a CFD computer code, the FLUENT was used to solve the equation for the heat transfer and pressure drop analyses in the fin-tube heat exchanger. The model drawing was created and meshed by using GAMBIT software. The heat transfer and pressure drop values of the vertical fin angle (θ = 0°) were provided to compare with variable inclined fin angles (θ = 5°, 10°, 15°, 20°, 25°, 30°). The heat transfer values were normalized to compare all cases. For inclined fin angle θ = 30°, which is the optimum angle, the maximum heat transfer enhancement per segment was obtained 1.42 W (the normalized value 105.24%), the maximum loss power associated with pressure drop per segment was only 0.54 mW.     

Condensation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of pure HFC-134a condensing inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786 m long helically coiled double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter and 8.3 mm inner diameter. The outer tube is made from smooth copper tubing of 23.2 mm outer diameter and 21.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305 mm. The pitch of coil is 35 mm. The test runs are done at average saturation condensing temperatures ranging between 40 and 50 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The pressure drop across the test section is directly measured by a differential pressure transducer. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy rejected from the test section. The effects of heat flux, mass flux and, condensation temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 33–53% and 29–46%, respectively. New correlations for the condensation heat transfer coefficient and pressure drop are proposed for practical applications.     

Effects of duct inclination angle on thermal entrance region of laminar and transition mixed convection

Experimental investigation was performed on the mixed convection heat transfer of thermal entrance region in an inclined rectangular duct for laminar and transition flow. Air flowed upwardly and downwardly with inclination angles from ?90° to 90°. The duct was made of duralumin plate and heated with uniform heat flux axially. The experiment was designed for determining the effects of inclination angles on the heat transfer coefficients and friction factors at seven orientations (θ = ? 90°, ?60°, ?30°, 0°, 30°, 60° and 90°), six Reynolds numbers (Re ≈ 420, 840, 1290, 1720, 2190 and 2630) within the range of Grashof numbers from 6.8 × 10³ to 4.1 × 10⁴. The optimum inclination angles that yielded the maximum heat transfer coefficients decreased from 30° to ?30° with the increase of Reynolds numbers from 420 to 1720. The heat transfer coefficients first increased with inclination angles up to a maximum value and then decreased. With further increase in Reynolds numbers, the heat transfer coefficients were nearly independent of inclination angles. The friction factors decreased with the increase of inclination angles from ?90° to 90° when Reynolds numbers ranged from 420 to 1290, and independent of inclination angles with higher Reynolds numbers.     

Experimental study of evaporation heat transfer of R-134a inside a corrugated tube with different tube inclinations

Experimental heat transfer studies during evaporation of R-134a inside a corrugated tube have been carried out. The corrugated tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of − 90° to + 90° and four mass velocities of 46, 81, 110 and 136 kg m^− 2 s^− 1 for each tube inclination angle during evaporation of R-134a. Data analysis demonstrate that the tube inclination angle, α, affects the boiling heat transfer coefficient in a significant manner. The effect of tube inclination angle, α, on heat transfer coefficient, h, is more prominent at low vapor quality and mass velocity. In the low vapor quality region, the heat transfer coefficient, h, for the + 90° inclined tube is about 62% more than that of the − 90° inclined tube. The results also showed that at all mass velocities, the highest average heat transfer coefficient were achieved for α = + 90°. An empirical correlation has also been developed to predict the heat transfer coefficient during flow boiling inside a corrugated tube with different tube inclinations.     

Radiative effects on natural convection in vertical convergent channels

Natural convection in air, in a convergent channel, uniformly heated at the principal walls, is experimentally investigated, in order to analyze the effects of the radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls inclination angle, the spacing between the walls, the heat flux, are given for two values of the wall emissivity. Flow visualization is carried out to show the peculiar pattern of the flow between the plates in several configurations. The comparison between two wall emissivity values, 0.10 and 0.90, shows that the effect of thermal radiation is more pronounced for larger convergence angles. For a wall emissivity equal to 0.90 and for small values of the minimum channel spacing, heat transfer in slightly convergent vertical channels is stronger than in a vertical parallel channel. Flow visualization points out a recirculating zone in the upper part of the channel for small values of the minimum channel spacing and for converging angles equal to 5° and 10°. Nusselt numbers and dimensionless maximum temperatures are then evaluated and correlated to the Rayleigh number, in the investigated range from 5 to 5 × 10⁸ and 0° ? θ ? 10°. A very good agreement between experimental data and correlations is observed for the dimensionless parameters based on the maximum channel spacing. Comparisons between experimental and numerical data are also performed and a good relationship is observed.     

Evaporation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of HFC-134a during evaporation inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786-m long helically coiled tube with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from copper tubing of 9.52 mm outer diameter and 7.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper tube into a spiral coil. The diameter of coil is 305 mm. The test run are done at average saturated evaporating temperatures ranging between 10 and 20 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The inlet quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of heat flux, mass flux and, evaporation temperature on the heat transfer coefficients and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the straight tube reported in the literature. New correlations for the convection heat transfer coefficient and pressure drop are proposed for practical applications.     

Heat transfer during condensation of moving steam in a narrow channel

The paper presents the results of experimental investigation of heat transfer and hydrodynamics during condensation of moving steam in a narrow channel of square cross-section 2 mm × 2 mm. The channel had a serpentine shape, the channel length was 660 mm. An experimental cell simulated conditions of heat transfer in the condenser of loop heat pipes. The steam velocity at the channel inlet ranged from 13 to 52 m/s, the pressure was 1 atm. The temperature of the cooling water varied from 70 to 95 °C. The annular flow pattern was noted in the whole range of the regime parameters. There was a clear boundary between the condensation zone and the zone occupied by the condensed phase downstream. Temperature has measured along the channel, and the heat-transfer coefficients have been determined. The coefficient values varied from 10,000 to 55,000 W/K m² depending on the steam velocity at the channel inlet and the cooling temperature. The efficiency of the condenser – heat exchanger has been investigated.     

Dryout characteristics during flow boiling of R134a in vertical circular minichannels

In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m² s to 600 kg/m² s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 °C and 32 °C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.     

Performance evaluation of mixed convection in an inclined square channel with uniform temperature walls

Numerical analyses were performed for the effect of inclined angle on the mixing flow in a square channel with uniform temperature walls (T_w = 30 °C) and inlet temperature (T₀ = 10 °C). Three-dimensional governing equations were solved numerically for Re = 100, Pr = 0.72 and various inclined angles (from ?90° to 90°). Three-dimensional behavior of fluid in a channel was examined for each angle. Thermal performance was evaluated using the relationship between Nusselt number ratio and pressure loss ratio with and without buoyancy induced flow as a parameter of inclined angles. High heat transfer and low pressure loss region was from ?15° to ?60° in thermal performance using mean Nusselt number ratio.     

Effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions

This study investigated the effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions. A total of 10 samples were tested with associated fin thickness (δ_f) of 0.115 mm and 0.25 mm, respectively. For a heat exchanger with two rows (N = 2) and fin pitch F_p of 1.41 mm, the effect of fin thickness on the heat transfer coefficient is more pronounced. The heat transfer coefficients for δ_f = 0.25 mm is about 5–50% higher than those for δ_f = 0.115 mm whereas the pressure drop for δ_f = 0.25 mm is about 5–20% higher. The unexpected difference in heat transfer coefficient subject to fin thickness is attributable to better interactions between the directed main flow and the swirled flow caused by the condensate droplet for δ_f = 0.25 mm. The maximum difference in heat transfer coefficients for N = 2 and F_p = 2.54 mm subject to the influence of fin thickness is reduced to about 20%, and there is no difference in heat transfer coefficient when the frontal velocity is above 3 m/s. For N ⩾ 4 and F_p = 2.54 mm, the influence of fin thickness on the heat transfer coefficients diminishes considerably. This is because of the presence of tube row, and the unsteady/vortex shedding feature at the down stream of wavy channel. Based on the present test results, a correlation is proposed to describe the air-side performance for wavy fin configurations, the mean deviations of the proposed heat transfer and friction correlations are 7.9% and 7.7%, respectively.     

Pressure drop characteristics of R134a during flow boiling in a single rectangular micro-channel

The pressure-drop characteristics during flow boiling in a single rectangular micro-channel with hydraulic diameter of 0.68 mm are presented. In the present study, pressure drop was studied at heat flux range of 7.63–49.46 kW/m², mass flux range of 600–1400 kg/m² s, and saturation temperature of 23, 27 and 31 °C. Experimental results indicated that the total pressure was dominated by frictional pressure drop. The increase of mass flux also increased the frictional pressure gradient, whereas the increase of saturation temperature reduced the frictional pressure gradient. In addition, heat flux also had an insignificant effect on the frictional the pressure gradient. A new correlation was also proposed for effective design of micro-channel heat exchanger.     

Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers

Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solution sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems.In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capacity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side.Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated.In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U_overall varied from 970 W/m² °C (170 Btu/h ft² °F) to 2270 W/m² °C (400 Btu/h ft² °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry.     

Experimental and theoretical study of critical heat flux in vertical upflow with inlet vapor void

This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a 2.5 mm × 5 mm vertical upflow channel that is heated along its 2.5 mm sidewall downstream of an adiabatic development section. Unlike most prior CHF studies, where the working fluid enters the channel in liquid state, the present study concerns saturated inlet conditions with finite vapor void. Temperature measurements and high-speed video imaging techniques are used to investigate the influence of the inlet vapor void on interfacial behavior at heat fluxes up to CHF as well during the CHF transient. The flow entering the heated portion of the channel consists of a thin liquid layer covering the entire perimeter surrounding a large central vapor core. Just prior to CHF, a fairly continuous wavy vapor layer begins to develop between the liquid layer covering the heated wall and the heated wall itself, resulting in a complex four-layer flow consisting of the liquid layer covering the insulated walls, the central vapor core, the now separated liquid layer adjacent to the heated wall, and the newly formed wavy vapor layer along the heated wall. This behavior in captured in a new separated control-volume-based model that facilities the determination of axial variations of thicknesses and mean velocities of the four layers. Incorporating the results of this model in a modified form of the Interfacial Lift-off CHF Model is shown to provide fairly good predictions of CHF data for mass velocities between 185 and 1600 kg/m² s, evidenced by a mean absolute error of 24.52%.     

Assessment of an active-cooling micro-channel heat sink device,using electro-osmotic flow

Non-uniform heat flux generated by microchips causes “hot spots” in very small areas on the microchip surface. These hot spots are generated by the logic blocks in the microchip bay; however, memory blocks generate lower heat flux on contrast. The goal of this research is to design, fabricate, and test an active cooling micro-channel heat sink device that can operate under atmospheric pressure while achieving high-heat dissipation rate with a reduced chip-backside volume, particularly for spot cooling applications. An experimental setup was assembled and electro-osmotic flow (EOF) was used thus eliminating high pressure pumping system. A flow rate of 82 μL/min was achieved at 400 V of applied EOF voltage. An increase in the cooling fluid (buffer) temperature of 9.6 °C, 29.9 °C, 54.3 °C, and 80.1 °C was achieved for 0.4 W, 1.2 W, 2.1 W, and 4 W of heating powers, respectively. The substrate temperature at the middle of the microchannel was below 80.5 °C for all input power values. The maximum increase in the cooling fluid temperature due to the joule heating was 4.5 °C for 400 V of applied EOF voltage. Numerical calculations of temperatures and flow were conducted and the results were compared to experimental data. Nusselt number (Nu) for the 4 W case reached a maximum of 5.48 at the channel entrance and decreased to reach 4.56 for the rest of the channel. Nu number for EOF was about 10% higher when compared to the pressure driven flow. It was found that using a shorter channel length and an EOF voltage in the range of 400–600 V allows application of a heat flux in the order of 10⁴ W/m², applicable to spot cooling. For elevated voltages, the velocity due to EOF increased, leading to an increase in total heat transfer for a fixed duration of time; however, the joule heating also got elevated with increase in voltage.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Development and tests of ammonia Miniature Loop Heat Pipes with cylindrical evaporators

Miniature Loop Heat Pipes (MLHPs) are an attractive object for development and investigation as quite a promising means for cooling powerful electronics operating in the temperature range from 50 to 100 °C. The paper generalizes and presents the results of development and tests of 15 different variants of ammonia MLHPs with cylindrical evaporators 5 and 6 mm in diameter, which have an active zone length of 20 mm and are equipped with titanium and nickel wicks. As a result of successive efforts aimed at increasing the MLHPs efficiency, it was possible to achieve values of the heat-transfer coefficient close to 162,000 W/m² °C at a value of the heat flux of about 100 × 10⁴ W/m². A maximum heat flux value of about 135 × 10⁴ W/m² was achieved at the heat-transfer coefficient equal approximately to 75,000 W/m² °C.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.