Experimental investigation of heat transfer in a packed duct with unequal wall temperatures

Wall-to-air forced convective heat transfer has been measured in a rectangular duct with unequal wall temperatures. The measurements were taken for a developing region in laminar flow. A uniform heat flux was supplied to the front wall of the duct. Heat transfer coefficients were measured in the empty and packed duct for various airflow rates and for different diameters of Raschig ring type packings. Measurements indicate that there is a considerable increase in the value of heat transfer coefficient for a packed duct over that for a duct with an empty flow passage.     

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.     

An experimental investigation of heat transfer characteristics for turbulent flow over staggered ribs in a square duct

An experimental study of developing and fully developed turbulent air flow in a square duct with two opposite rib-roughened walls in which the ribs are attached in a staggered fashion was conducted to determine the heat transfer characteristics. The rib height-to-hydraulic diameter ratio (e/D_H) was 0.19, the rib pitch-to-height ratio (p/e) was 5.31. The streamwise temperature distribution was measured, and a law of the wall for the thermal boundary layer at each free-stream turbulence level was obtained. The effects of free-stream turbulence intensity with variations of 4–11% on heat transfer coefficients were also examined. Finally, the relationship between Nusselt number and Reynolds number was correlated. The results might be used in the design of turbine blade cooling channels.     

Experimental investigation of heat transfer and pressure drop characteristics of flow through spirally fluted tubes

Spirally fluted tubes are used extensively in the design of tubular heat exchangers. In previous investigations, results for tubes with flute depths e/D_vi < 0.2 were reported, with most correlations applicable for Re ≥ 5000. This paper presents the results of an experimental investigation of the heat transfer and pressure drop characteristics of spirally fluted tubes with the following tube and flow parameter ranges: flute depth e/D_vi = 0.1−0.4, flute pitch p/D_vi = 0.4−7.3, helix angle θ/90° = 0.3−0.65, Re = 500−80,000, and Pr = 2−7. The heat transfer coefficients inside the fluted tube were obtained from measured values of the overall heat transfer coefficient using a nonlinear regression scheme. The friction factor data obtained consisted of 507 data points. The proposed correlation for the friction factor predicts 96% of the database within ±20%. The heat transfer correlation for the range 500 ≤ Re ≤ 5000 predicts 76% of the database (178 data points) within ±20%, and the correlation for the higher Re range predicts 97% of the 342 data points within ±20%. Comparison of heat transfer and friction data show that these tubes are most effective in the laminar and transition flow regimes. The present results show that the increase of flute depth in the range considered does not improve heat transfer.     

Experimental study of heat and mass transfer from a horizontal cylinder in downward air-water mist flow with blockage effect

An experimental study was made on convective heat and mass transfer from a horizontal heated cylinder in a downward flow of air-water mist at a blockage ratio of 0.4. The measured local heat transfer coefficients agree fairly well with the authors' numerical solutions obtained previously for the front surface of a cylinder over the ranges mass flow ratio 0–4.5×10⁻², a temperature difference between the cylinder and air 10–43 K, gas Reynolds number (7.9–23)×10³, Rosin-Rammler size parameter 105–168 μm, and dispersion parameter 3.4–3.7. Heat transfer augmentation, two-pahse to single-phase of greater than 19 was attained at the forward stagnation point. For heat transfer in the rear part of the cylinder, an empirical formula is derived by taking into account the dimensionless governing variables, that is, coolant-feed and evaporation parameters.     

Computational analysis of heat transfer and pressure drop performance for internally finned tubes with three different longitudinal wavy fins

Turbulent pressure drop and heat transfer characteristics in tubes with three different kinds of internally longitudinal fin patterns (interrupted wavy, sinusoidal wavy and plain) are numerically investigated for Re = 904–4,520. The channel velocity, temperature, and turbulence fields are obtained to discern the mechanisms of heat transfer enhancement. Numerical results indicate that the steady and spatially periodic growth and disruption of cross-sectional vortices occur near the tube/fin walls along the streamwise locations. The thermal boundary layers near the tube/fin surfaces are thereby periodically interrupted, with heat transfer near the recirculation zones being enhanced. The overall heat transfer coefficients in wavy channels are higher than those in a plain fin channel, while with larger pressure drop penalties. At the same waviness, the interrupted wavy fin tube could enhance heat transfer by 72–90%, with more than 2–4 times of pressure drop penalty. Among the fins studied, the sinusoidal wavy fin has the best comprehensive performance.     

Darcy-Forchheimer natural,forced and mixed convection heat transfer in non-Newtonian power-law fluid-saturated porous media

The governing equation for Darcy-Forchheimer flow of non-Newtonian inelastic power-law fluid through porous media has been derived from first principles. Using this equation, the problem of Darcy-Forchheimer natural, forced, and mixed convection within the porous media saturated with a power-law fluid has been solved using the approximate integral method. It is observed that a similarity solution exists specifically for only the case of an isothermal vertical flat plate embedded in the porous media. The results based on the approximate method, when compared with existing exact solutions show an agreement of within a maximum error bound of 2.5%.Nomenclature A cross-sectional area - b _i coefficient in the chosen temperature profile - B ₁ coefficient in the profile for the dimensionless boundary layer thickness - C coefficient in the modified Forchheimer term for power-law fluids - C ₁ coefficient in the Oseen approximation which depends essentially on pore geometry - C _i coefficient depending essentially on pore geometry - C _D drag coefficient - C _t coefficient in the expression forK ^* - d particle diameter (for irregular shaped particles, it is characteristic length for average-size particle) - f _p resistance or drag on a single particle - F _R total resistance to flow offered byN particles in the porous media - g acceleration due to gravity - g _x component of the acceleration due to gravity in thex-direction - Grashof number based on permeability for power-law fluids - K intrinsic permeability of the porous media - K ^* modified permeability of the porous media for flow of power-law fluids - l _c characteristic length - m exponent in the gravity field - n power-law index of the inelastic non-Newtonian fluid - N total number of particles - Nux,_D,F local Nusselt number for Darcy forced convection flow - Nux,_D-F,F local Nusselt number for Darcy-Forchheimer forced convection flow - Nux,_D,M local Nusselt number for Darcy mixed convection flow - Nux,_D-F,M local Nusselt number for Darcy-Forchheimer mixed convection flow - Nux,_D,N local Nusselt number for Darcy natural convection flow - Nux,_D-F,N local Nusselt number for Darcy-Forchheimer natural convection flow - pressure - p exponent in the wall temperature variation - _Pe c characteristic Péclet number - _Pe x local Péclet number for forced convection flow - _Pe x modified local Péclet number for mixed convection flow - _Ra c characteristic Rayleigh number - _Ra x local Rayleigh number for Darcy natural convection flow - _Ra x local Rayleigh number for Darcy-Forchheimer natural convection flow - Re convectional Reynolds number for power-law fluids - Reynolds number based on permeability for power-law fluids - T temperature - T _e ambient constant temperature - T w,_ref constant reference wall surface temperature - T _w(X) variable wall surface temperature - T _w temperature difference equal toT w,_ref–T _e - T ₁ term in the Darcy-Forchheimer natural convection regime for Newtonian fluids - T ₂ term in the Darcy-Forchheimer natural convection regime for non-Newtonian fluids (first approximation) - T _N term in the Darcy/Forchheimer natural convection regime for non-Newtonian fluids (second approximation) - u Darcian or superficial velocity - u ₁ dimensionless velocity profile - u _e external forced convection flow velocity - u _s seepage velocity (local average velocity of flow around the particle) - u _w wall slip velocity - U c _M characteristic velocity for mixed convection - U c _N characteristic velocity for natural convection - x, y boundary-layer coordinates - x ₁,y ₁ dimensionless boundary layer coordinates - X coefficient which is a function of flow behaviour indexn for power-law fluids - effective thermal diffusivity of the porous medium - shape factor which takes a value of/4 for spheres - shape factor which takes a value of/6 for spheres - ₀ expansion coefficient of the fluid - _T boundary-layer thickness - T ₁ dimensionless boundary layer thickness - porosity of the medium - similarity variable - dimensionless temperature difference - coefficient which is a function of the geometry of the porous media (it takes a value of 3 for a single sphere in an infinite fluid) - ₀ viscosity of Newtonian fluid - ^* fluid consistency of the inelastic non-Newtonian power-law fluid - constant equal toX(2 ^2–n )/ - density of the fluid - dimensionless wall temperature difference     

Experimental study on the pressure drop and heat transfer characteristics of tubes with internal wave-like longitudinal fins

Experiments were performed to determine the heat transfer and pressure drop characteristics in the entrance and fully developed regions of tubes with internal wave-like longitudinal fins. The test tube has a double-pipe structure, with the inner tube as an insertion. The wave-like fins are in the annulus and span its full width. Experiments were conducted for two cases: one with the inner tube blocked (no air flowing through it) and the other with the inner tube unblocked. The outer tube was electrically heated. Local and average heat transfer coefficients and friction factors were measured. The friction factor and Nusselt number correlations in the fully developed region were obtained in the Reynolds number range of 9×10² to 3.5×10³. It has been found that the wave-like fins enhance heat transfer significantly with the blocked case being superior. In addition, the in-tube heat transfer process is characterized by an earlier transition from laminar to turbulent flow and Reynolds number-dependent thermal entrance length. Received on 12 May 1998     

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.     

Experimental investigation on condensation heat transfer and pressure drop of R134a in a plate heat exchanger

Condensation heat transfer of R134a in a vertical plate heat exchanger was investigated experimentally. The local heat transfer coefficients are determined by means of the measured local wall temperatures. A differential energy balance model is developed for data evaluation. It is found that the correlation proposed by Shah using Ψ and Z factors is suitable for condensation in plate heat exchangers and is adopted to fit the measured data.     

Experimental investigation of ice slurry flow pressure drop in horizontal tubes

Pressure drop behaviour of ice slurry based on ethanol–water mixture in circular horizontal tubes has been experimentally investigated. The secondary fluid was prepared by mixing ethyl alcohol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature ?4.4 °C). The pressure drop tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 30% depending on test conditions. Results from flow tests reveal much higher pressure drop for higher ice concentrations and higher velocities in comparison to the single phase flow. However for ice concentrations of 15% and higher, certain velocity exists at which ice slurry pressure drop is same or even lower than for single phase flow. It seems that higher ice concentration delay flow pattern transition moment (from laminar to turbulent) toward higher velocities. In addition experimental results for pressure drop were compared to the analytical results, based on Poiseulle and Buckingham–Reiner models for laminar flow, Blasius, Darby and Melson, Dodge and Metzner, Steffe and Tomita for turbulent region and general correlation of Kitanovski which is valid for both flow regimes. For laminar flow and low buoyancy numbers Buckingham–Reiner method gives good agreement with experimental results while for turbulent flow best fit is provided with Dodge–Metzner and Tomita methods.Furthermore, for transport purposes it has been shown that ice mass fraction of 20% offers best ratio of ice slurry transport capability and required pumping power.     

Heat transfer and pressure drop characteristics of turbulent flow in a circular tube fitted with regularly spaced twisted-tape elements

The results of an experimentalnvestigation of turbulent flow heat transfer and pressure drop characteristics in a circular tube fitted with regularly spaced twisted-tape elements connected by thin circular rods are reported. The characteristics are governed by Reynolds number, Prandtl number, twist ratio, space ratio, and rod-to-tube diameter ratio. Correlations for friction factor and Nusselt number are also reported. It is shown that on the basis of both constant pumping power and constant heat duty, regularly spaced twisted-tape elements do not perform better than full-length twisted tapes.     

Augmented heat transfer and pressure drop of strip-type inserts in the small tubes

An experiment is carried out to investigate the characteristics of the augmentation of heat transfer and pressure drop by different strip-type inserts in small tube having an inside diameter of 2.0 mm. The effects of the imposed wall heat flux, mass flux, strip inserts with various configurations (heights, widths, pitches) on the measured augmentative heat transfer and pressure drop are examined in detail. In order to obtain insight into the fluid flow phenomena, flow visualization was also made to observe the detailed fluid flow characteristics of the present tubes inserted with strip-type inserts. In addition, comparisons are made with a plain tube having the same length, heat transfer area and experimental conditions. The measured heat transfer coefficients and pressure drops for this small pipe are also emphasized to compare with those for larger pipes. Furthermore, in order to compare results from the different configurations of strip-type inserts, several enhancement factors and performance ratios are defined to account for the effects of augmentation. Moreover, correlation equations for the heat transfer coefficient and pressure drop of the present study are proposed.The financial support extended by the National Science Council of the Republic of China through grant No. NSC-89-2212-E-230-004.     

Experimental investigation of TiO2/water nanofluid laminar forced convective heat transfer through helical coiled tube

Coiled tubes and nanofludics are two significant techniques to enhance the heat transfer ability of thermal equipments. The forced convective heat transfer and the pressure drop of nanofluid inside straight tube and helical coiled one with a constant wall heat flux were studied experimentally. Distilled water was used as a host fluid and Nanofluids of aqueous TiO₂ nanoparticles (50 nm) suspensions were prepared in various volume concentrations of 0.25–2 %. The heat transfer coefficient of nanofluids is obtained for different nanoparticle concentrations as well as various Reynolds numbers. The experiments covered a range of Reynolds number of 500–4,500. The results show the considerable enhancement of heat transfer rate, which is due to the nanoparticles present in the fluid. Heat transfer coefficient increases by increasing the volume concentration of nanoparticles as well as Reynolds number. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, both convective heat transfer coefficient and the pressure drop of fluid grow considerably. Also, the thermal performance factors for tested nanofluids are greater than unity and the maximum thermal performance factor of 3.72 is found with the use of 2.0 % volume concentration of nanofluid at Reynolds number of 1,750.     

Heat transfer and pressure drop in tubes with short turbulators

The results of investigations concerning heat transfer and pressure drop in a tube with short single-turn turbulators in form of a twisted tape are presented here. The experiments were carried out in the range: 1300≤Re≤8000 4,3≤Pr≤7,0, 2,9≤S/d≤4,4 with water as the test fluid. The results were compared with those obtained when investigating heat transfer and flow resistance in a tube with a turbulator installed all along the tube. It was demonstrated that in certain conditions, assuming thatQ=idem,F=idem andS=idem, the power demand for pumping medium can be reduced.     

Experimental study of heat transfer characteristics due to confined impinging two-dimensional jets

Experimental results are presented for characteristics of impingement heat transfer caused by three slot jets. Experimental values were obtained for the dimensionless distance H = 0.5−3, dimensionless pitch P = 6−16, and Reynolds number Re = 500−8000. For laminar impinging flow, they were compared with numerical results. For turbulent impinging flow, two peaks of the local Nusselt number were obtained behind the second nozzle. The position of the second peak approached the nozzle as the space between nozzle and impinged surface decreased. The average Nusselt number between the central and second nozzles was determined from the ratio P/H and the Reynolds number based on the pitch of the nozzles.     

Quantitative infrared investigation of local heat transfer in a circular finned tube heat exchanger assembly

This work deals with the local heat transfer coefficient evaluation over the fin of the second row of a staggered circular finned tube heat exchanger assembly. The coefficient distribution is determined by using a transient technique and by calculating the energy balance during the fin cooling. The calculation model takes into account radiation with the surrounding and lateral heat conduction into the material. The method uses infrared measurements and integration between time bounds that depend on space. It is proposed to choose the integration bounds with an original criterion based on local heat transfer. Validation is performed on the reference case consisting in a thin plate in an aerodynamically and thermally developing channel flow. Then, distributions of Nusselt number on the circular fin are presented for several Reynolds numbers. The high resolution of the whole method and set-up allow detecting thermal imprints of developing horseshoe vortices. These imprints are analyzed by following their angular evolution around the tube.     

Experimental study on forced convective heat transfer of flowing gaseous solid suspension at high temperature

The results of an experimental study of the forced convective heat transfer to helium-graphite suspension at high temperatures up to 1173K are presented. Entering gas Reynolds number ranges from 1.0 x 10⁴ to 2.0 x 10⁴ and the particle loading ratio reaches about 4. The ratio of the Nusselt number of the suspension to that of gas alone increases considerably in a range of high loading ratios as the wall temperature increases. Subsequently, two kinds of turbulence promoters (200 and 400 mm pitch twisted tapes) are inserted in the flowing gaseous solid suspensions to make use of the large inertia forces of particles. The current results show that the local heat fluxes with use of the tapes increase significantly with the rise in the wall temperature owing to the radiative effect of the particulate phase.     

Mixed convective heat and mass transfer in a non-Newtonian fluid at a peristaltic surface with temperature-dependent viscosity

We investigate the problem of the unsteady mixed convection peristaltic mechanism. The flow includes a temperature-dependent viscosity with thermal diffusion and diffusion-thermo effects. The peristaltic flow is between two vertical walls, one of which is deformed in the shape of traveling transversal waves exactly like peristaltic pumping and the other of which is a parallel flat plate wall. The equations of momentum, energy, and concentration are subject to a set of appropriate boundary conditions by assuming that the solution consists of two parts: a mean part and a perturbed part. The solution of the perturbed part has been obtained by using the long-wave approximation. The mean part has been solved and coincides with the approximation of Ostrach. The mean part (zeroth order), the first order, and the total solution of the problem have been evaluated numerically for several sets of values of the parameters entering the problem. The skin friction, and the rate of heat and mass transfer at the walls are obtained and illustrated graphically.     

Experimental study on the forced convective flow in a channel with heated blocks in tandem

This study experimentally examines the forced convective flow over two sequentially heated blocks mounted on one principal wall of a channel. The experiments, involving mass transfer, were carried out via the naphthalene sublimation technique (NST). By virtue of the analogy between heat and mass transfer, the results can then be converted to determine the heat transfer. In the experiments, the block spacings were set at 2, 4, 6, 8, 12, 16, and 22 and the Reynolds numbers were set at 1300 and 10⁴ which correspond to the laminar and the turbulent convective flow cases, respectively. Results show that the Sherwood number increases or decreases monotonically along the block surfaces in the laminar convection cases; while the hump and sharp increase in the Sherwood number can be found in the turbulent convection cases. This is attributed to the reattachment of the separating bubble and the flow impingement, respectively. Comparison between the experimental and numerical results is made and the effect of the block spacing on heat transfer is discussed.