Heat Transfer Enhancement Levels Research Articles

Experimental and numerical investigations on the effect of a novel internal surface micro-grooving toward improving convective heat transfer performance of tube heat exchangers

In the present work, the use of passive heat transfer enhancement technique through surface alteration was explored. The enhancement was achieved through internal conduit surface micro-grooving using a new apparatus that was developed by modifying a magnetic abrasive finishing technique. A new surface profile was created and later verified using optical and laser profiler measurements. The new profile was numerically investigated to compare the heat transfer and hydrodynamic performance against other profiles that have been studied in the literature. It was found from the results that the new profile shows much higher heat transfer improvement and comparable pressure loss to the previously investigated profiles (i.e., square, rectangular, trapezoidal, and circular). Overall, the new groove geometry provides the highest performance followed by the circular, triangular, curvy, square, and rectangular grooves. Also, the results indicate that designs with a smooth profile performs better than those with sharp edges, owing to the elimination of stationary fluid spots within the grooves. The effectiveness between the profiles was compared based on the level of heat transfer enhancement against the flow penalty. Experimental validation was further conducted for the experimentally generated groove. The results revealed that a relatively small surface temperature drop was obtained, corresponding to a slight improvement in heat transfer. This confirms the results generated by the simulation that groove size plays a major role in attaining significant improvement in heat transfer.

Thermal characterization of a heating cylinder under ultrasonic effects

Ultrasonic effects were investigated on the heat transfer characteristics of a vertical cylinder releasing a constant heat flux of 14,150 W/m2 in distilled water. A triple-frequency ultrasonic transducer was installed at the center of the lateral wall of the cubic test section to emit 40, 80, and 120 kHz ultrasonic waves. The thermal characterization was investigated by varying the distance (5, 10, and 15 cm) between the ultrasonic transducer and the heating surface. The surface temperature of the heater, detected using 12 thin-leaf thermocouples, decreased rapidly because of the ultrasonic effect, leading to an increase in the Nusselt number. An infrared camera and a video camera were used for the visualization and investigation of acoustic streaming and acoustic cavitation. The results showed that ultrasound enhanced heat transfer to the maximum level of 82.4% using 40 kHz waves at a distance of 5 cm. At distances of 10 and 15 cm, 120 kHz ultrasonic waves produced the highest heat transfer enhancement levels of 74.2% and 59.7%, respectively. The heat transfer enhancement ratio decreased with increasing incident angle of the wave beam as well as with the distance between the ultrasonic transducer and the heating surface. The key mechanisms leading to the enhancement of heat transfer in the near and far fields were identified and discussed. Finally, an accurate prediction formula was proposed for the overall heat transfer enhancement ratio, depending on the ultrasonic frequency and distance.

A target-evaluation method for heat exchanger network optimisation with heat transfer enhancement

Energy conservation and efficiency improvement are critical problems for industrial process systems. Heat exchanger network retrofit has emerged as a powerful tool for energy-saving and Heat Integration, but the topology modification with new heat exchangers and re-piping is expensive and demands a long payback period. In this study, heat transfer enhancement is deployed in the heat exchanger network retrofit to avoid network topology modification. A target-evaluation method is proposed for heat exchanger network retrofit with the consideration of the thermal efficiency and the level of heat transfer enhancement. Two case studies are conducted to demonstrate the proposed method. The energy-saving of case 1 is 10.6752 MW, which is 13.3% of the original utility loads; 8.0303 MW heat loads are saved in case 2, which is 14.7% of the original utility loads. The results of return on investment indicate that enhancement of the best candidate heat exchanger can bring more energy-saving and better economic efficiency with fewer modifications to the existing heat exchangers, which is also confirmed by sensitivity analysis. The lower temperature effectiveness of the heat exchanger shows great potential for thermal efficiency improvement. The present results highlight the energy-saving effects that heat transfer enhancement has on the heat exchanger network retrofit. The proposed two-stage method supports the further development of energy conservation in different industries.

The Cooling Performance of Mixed Convection in a Ventilated Enclosure With Different Ports Configurations

Abstract Mixed convection heat transfer in a vertically oriented air-cooled square enclosure is simulated numerically using the finite volume method. The vertical left wall being heated and all others are considered insulated. The effects of six opposite and staggered inlet/outlet openings' locations over the top and bottom walls are investigated, with different sizes of the opening height (0.05 ≤d/H≤ 0.3). The objective is to figure out the better size and location of inlet and outlet to acquire more efficient cooling system in the enclosure by maximizing the heat removal rate. This is conducted over ranges of the governing parameters, which are the Richardson number (0 ≤Ri≤ 30) and the Reynolds number (50 ≤Re≤ 250). The results show that the level of heat transfer enhancement increases with increasing the opening height, with an optimal size of d/H = 0.25 for obtaining maximum heat removal, for all Richardson and Reynolds numbers. The results also indicate that the higher heat dissipation occurs when the cold air is injected vertically near the hot wall and exits the enclosure from the opposite or staggered outlet, for all Richardson and Reynolds numbers.

Experimental and Numerical Study of Chord-Wise Eight-Passage Serpentine Cooling Design for Eliminating the Coriolis Force Adverse Effect on Heat Transfer

Abstract Gas turbine blades are equipped with internal cooling channels which are connected by 180 deg bends. Due to combined effects of Coriolis force and centrifugal buoyancy force, the heat transfer increases on the trailing side (pressure side) and decreases on the leading side (suction side) for radially outward flow. The trend in heat transfer is opposite for radially inward flow. This configuration leads to nonuniform blade temperature which in unfavorable for blade lifespan. This paper presents a novel eight-passage serpentine design, where passages are arranged along the chord of the blade, to rectify the negative effects of Coriolis force on heat transfer and is an extension four- and six-passage smooth channel studies conducted by the authors earlier. Transient liquid crystal thermography (TLCT) is carried out for detailed measurement of heat transfer coefficients. Heat transfer experiments were performed for Reynolds numbers between 14,264 and 83,616 under stationary conditions. For experiments under rotation, non-dimensional Rotation number is set as 0.05. Heat transfer enhancement levels of nearly twice the Dittus–Boelter correlation (for developed flow in smooth tubes) are obtained under stationary conditions. Under rotation, it is seen that the heat transfer enhancement levels on the leading and trailing sides are similar to each other and also with the stationary condition. Some differences in heat transfer are observed on local level, when rotation cases are compared against the stationary cases. Numerically predicted flow field is presented to support the experimental findings.

Investigation of Thermohydraulic Characteristics of Vortex Chambers

Local flow swirling is used in power facilities and other technical devices. It is an effective means for acting on the air flow structure and can enhance heat transfer. A swirled flow in axisymmetric channels belongs to the group of 3D flows in the field of centrifugal mass forces. It is characterized by the ratio of two (axial and rotational) or, in some cases, three components of the velocity, the presence of transverse and longitudinal pressure gradients, and high turbulent fluctuations that bring about certain difficulties in the investigation of processes occurring in a swirled flow and it complicates detection of their regularities. Therefore, it is proposed to install a vortex chamber (VC) at the leading edge of a blade or vane of a high-temperature gas turbine. Temperature conditions and flow capacity of VC models were studied by a calorimetric method in a liquid-metal thermostat. The regularities of heat transfer rate on the surface of the cooling channels were determined depending on the number and diameter of inlet (supply) and outlet holes for various pressure drops. The VK designs were optimized considering the effect of their geometry on the formation of various swirl flow structures with different levels of heat transfer enhancement. An analysis was performed with account taken of the throughput capacity of the models. The criterial dependences of the Nusselt number vs. Reynolds number Re were obtained for three VC designs for direct- or reverse-flow direction. The highest heat fluxes were observed on the section of coolant supply via holes, which is explained by a high velocity of the initial flow swirling. Flow swirl breakage and cooling air heating are the cause of a decrease in the relative heat transfer coefficient $${\bar {\alpha }}{\text{.}}$$ The self-similarity mode is observed at a pressure ratio across the vortex chamber above 1.4. The thermal problem was solved using data of the hydraulic tests of the models with air blowing under isothermal conditions. The results of experimental studies can be included in the data bank of heat and mass transfer software products to reduce the labor intensity and time in the development of a cooling system for blades/vanes of high-temperature gas turbines.

Thermal-Hydraulic Performance Enhancement by the Combination of Rectangular Winglet Pair and V-Shaped Dimples

Abstract This paper presents numerical study on heat transfer enhancement due to the combination of rectangular winglet pair with V-dimples in an array-type arrangement. Array of rectangular winglet pairs results in heat transfer enhancement, however, at a cost of significant pressure drop, resulting in reduced thermal-hydraulic performance (THP). On the other hand, dimples are associated with lower heat transfer enhancement levels at relatively lower pumping power penalty. To this end, a combination of rectangular winglet pair and V-shaped dimples has been studied in this paper, where the arrangements were intended to achieve enhanced thermal-hydraulic performance. Three different configurations, namely, rectangular winglet pair, rectangular winglet pair with one V-dimple between two consecutive winglets, and rectangular winglet pair with two V-dimples packed in a pitch, are studied here. The variation of heat transfer enhancement, pressure drop gain, and THP with respect to winglet-to-winglet (S) spacing variation for rectangular winglet pair and rectangular winglet pair with one V-dimple configuration is presented at a Reynolds number of 25,000. The THP of the rectangular winglet pair configuration decreases up to S/H equal to 2.5 and then increases (H: channel height). For rectangular winglet pair with one V-dimple, three values of winglet-to-dimple (P) spacings are analyzed. For fixed S/H, the highest P/H configuration provided highest heat transfer enhancement and THP. Among the three configurations studied, rectangular winglet pair with two V-dimples resulted in the highest thermal-hydraulic performance.

Thermal performance enhancement in a wedge duct with in-line pin fins combined with vortex generators

Purpose The purpose of this paper is to enhance the heat transfer and thermal performance in the trailing edge region of the vane with vortex generators (VGs). Design/methodology/approach This numerical study presents the enhancement of thermal performance in the trailing part of a gas turbine blade. In the trailing part, generally, pin fins are used either in staggered or in-line arrangements to enhance the heat transfer. In this study, based on the idea from heat exchangers, pin fins are combined with VGs. A pair of VGs is embedded in the boundary layer upstream of each pin fin in the first row of the pin fin array having an in-line configuration. The effects of the VG angle relative to the streamwise direction and streamwise distance between the pin fin and VGs are investigated at various Reynolds numbers. Findings The results indicated that the endwall heat transfer is enhanced with the addition of VGs and the heat transfer from the surfaces of the pin fins. The level of heat transfer enhancement compared to the case without VGs is more significant at high Reynolds number. The surfaces of the VGs also show a significant amount of heat transfer. Study of the angle of the attack suggested that a high angle of attack is more appropriate for pin fin cooling enhancement whereas an intermediate gap between the VGs and pin fins shows considerable improvement of thermal performance compared to the small and large gaps. The phenomenon of heat transfer augmentation with the VGs is demonstrated by the flow field. It shows that the enhancement of heat transfer is governed by the mixing of the flow as a result of the interaction of vortices generated by the VGs and pin fins. Originality/value VGs are used to disturb the thermal boundary layer. It shows that heat transfer is augmented as a result of the interaction of vortices associated with VGs and pin fins.

The flow and heat transfer characteristics in a rectangular channel with convergent and divergent slit ribs

Described in this paper is a numerical investigation on the concept for improving thermal performance of internal cooling by placing the convergent and divergent slit ribs. Five different geometrical models are investigated, including the ribs of rectangular slits and trapezoidal slits with different convergent and divergent angles. The effects of slit shape and its convergent or divergent angles on thermal performance of internal cooling are evaluated with the Reynolds numbers ranging from 10,000 to 25,000. Results obtained show that the turbulence intensity in cases with the smallest angle trapezoidal slits are in the highest level, which produces the highest level of heat transfer enhancement and highest level of pressure loss. The thermal performance index, which comprehensively evaluates the thermal performance of internal cooling, shows that the thermal performance in cases with the smallest angle trapezoidal slits are in the highest level due to the increased heat transfer enhancement and limited increase of pressure loss.

Rib Turbulator Heat Transfer Enhancements at Very High Reynolds Numbers

High-pressure stage gas turbine blades feature serpentine passages where rib turbulators are installed to enhance heat transfer between the relatively colder air bled off from the compressor and the hot internal walls. Most of the prior studies have been restricted to Reynolds number of 90,000 and several studies have been carried out to determine geometrically optimized parameters for achieving high levels of heat transfer in this range of Reynolds number. However, for land-based power generation gas turbines, the Reynolds numbers are significantly high and vary between 105 and 106. The present study is targeted toward these high Reynolds numbers where traditional rib turbulator shapes and prescribed optimum geometrical parameters have been investigated experimentally. A steady-state liquid crystal thermography technique is employed for measurement of detailed heat transfer coefficient. Five different rib configurations, viz., 45 deg, V-shaped, inverse V-shaped, W-shaped, and M-shaped have been investigated for Reynolds numbers ranging from 150,000 to 400,000. The ribs were installed on two opposite walls of a straight duct with an aspect ratio of unity. For very high Reynolds numbers, the heat transfer enhancement levels for different rib shapes varied between 1.4 and 1.7 and the thermal hydraulic performance was found to be less than unity.

Multi-Pass Serpentine Cooling Designs for Negating Coriolis Force Effect on Heat Transfer: Smooth Channels

The combined action of Coriolis and centrifugal buoyancy forces results in nonuniform heat transfer coefficient on pressure and suction side internal walls, hence leading to nonuniform metal temperatures and increased thermal stresses. The present study addresses the problem of nonuniform heat transfer distribution due to rotation effect and proposes novel designs for serpentine cooling passages, which are arranged along the chord of the blade. The two configurations were four-passage and six-passage serpentine smooth channels. Detailed heat transfer coefficients were measured using transient liquid crystal thermography under stationary and rotating conditions. Heat transfer experiments were carried out for Reynolds numbers ranging from 12,294 to 85,000 under stationary conditions. Rotation experiments were carried out for the Rotation numbers of 0.05 and 0.11. Heat transfer enhancement levels of approximately two times the Dittus–Boelter correlation (for developed flow in smooth tubes) were obtained under stationary conditions. Under rotating conditions, we found that the four-passage configuration had slightly lower heat transfer compared with the stationary case, and the six-passage configuration had higher heat transfer on both the leading and trailing sides compared with the stationary case. The leading and trailing side heat transfer characteristics were near-similar to each other for both the configurations, and the rotating heat transfer was near-similar to the stationary condition heat transfer.

Natural convection in symmetrically heated vertical channels

The study describes natural convection through a vertical channel, open on four sides and bound by two isothermal walls. Both balancing and infrared experimental investigations were performed in air (Pr = 0.71) to estimate the impact of channel width s and wall-to-ambient temperature difference (tw - t∞) on natural convective heat transfer, the formation of air flow patterns and the rate of flow pattern formation. The study was conducted on two parallel vertical plates of height H = 0.5 m and width B = 0.25 m, with the heated surfaces facing each other, thus creating peripherally open channels of different widths s = 0.045, 0.08, 0.180 and ∞ m. The surface temperature tw, identical for both heating plates, was changed every 10 K and set at tw = 40, 50, 60, 70 and 80 °C, while the ambient temperature was maintained within the 18–25 °C range. In the balance method, heat fluxes were determined based on measurements of voltage and electric current supplying the heaters placed inside the walls. In the gradient method, the heat fluxes were calculated from the temperature distribution in air, within a plane perpendicular to the heating plate surfaces. Temperature fields were visualized using a plastic detecting mesh and a thermal imaging camera. The distribution of temperature tx,y, and its gradient at the walls dt/dx|x=0,y were obtained at different heights y along the channels. The gradient values obtained and the results, presented as dependencies of the Nusselt number on the Rayleigh number, indicate that the channel width has a significant impact on heat transfer. Compared to the vertical plate s = ∞, the following levels of convective heat transfer enhancement were observed: 29.5% (s = 0.045 m), 38.8% (s = 0.085 m) and 61.6% (s = 0.180 m).

Effect of bleed hole on internal flow and heat transfer in matrix cooling channel

Matrix is an internal cooling scheme which has advantages of stronger heat transfer and better structural strength than conventional serpentine ribbed channels in gas turbine blades. In some compound cooling designs, coolant is extracted from the internal channel to the outer chamber to realize impingement. This paper analyzes the effect of bleed extraction on the flow field and heat transfer in matrix cooling channel by numerical simulations. RANS method is employed to present detailed flow field and heat transfer distribution. The investigated matrix channel is comprised of eight sub-channels which form two layers, the rib angle is 45° and the sub-channel aspect ratio is 1:1. The results indicate that heat transfer augmentation is caused by the large-scale streamwise vortexes which dominate the sub-channel flow. Extraction can create extremely high heat transfer enhancement level around bleed holes and alter the flow structures in the sub-channel. Cases of different diameters, positions and arrangements of holes are compared and the result shows that the bleed position has the most significant effect on the internal flow and heat transfer. Bleed holes between the sub-channel centerline and rib-side wall can provide strong heat transfer and good thermal performance. The internal matrix cooling capacity can be maintained with 25% mass extraction. Although the thermal performance of matrix channel can be improved by bleed holes, it is lower than most of the conventional cooling methods unless the baseline matrix structure was modified.

MHD Flow of Cu-Al<sub>2</sub>O<sub>3</sub>/Water Hybrid Nanofluid in Porous Channel: Analysis of Entropy Generation

In this investigation, a magnetohydrodynamic (MHD) flow of AlO /water nanofluid and Cu-AlO /water hybrid nanofluid through a porous channel is analyzed in the presence of a transverse magnetic field. An exact solution of the governing equations has been obtained in closed form. The entropy generation number and the Bejan number are also obtained. The influences of each of the governing parameters on velocity, temperature, entropy generation and Bejan number are displayed graphically and the physical aspects are discussed. In addition, a comparison of the heat transfer enhancement level due to the suspension of AlO and Cu nanoparticles in water as regular nanofluids and as hybrid Cu-AlO /water nanofluid is reported.

Numerical simulation of FENE-P viscoelastic fluids flow and heat transfer in grooved channel with rectangular cavities

The forced convection heat transfer for non-Newtonian viscoelastic fluids obeying the FENE-P model in a parallel-plate channel with transverse rectangular cavities is carried out numerically using ANSYS-POLYFLOW code. The flow investigated is assumed to be two-dimensional, incompressible, laminar and steady. The flow behavior and temperature distribution influenced by the re-circulation caused by the variation of cross-section area along the stream wise direction have been studied. The constant heat flux condition has been applied and the effects of the different parameters, such as the aspect ratio of channel cavities (AR = 0.25, 0.5), the Reynolds number (Re = 25, 250, and 500), the fluid elasticity defined by the Weissenberg number (We), and the extensibility parameter of the model (L 2), on heat transfer characteristics have been explored for channels of three successive cavities configuration. Different levels of heat transfer enhancement were obtained and discussed.

Investigation of heat transfer enhancement by electrohydrodynamics in a double-wall-heated channel

In this study, heat transfer enhancement by electrohydrodynamics (EHD) in a rectangular double-wall-heated channel with an emitting electrode pair is investigated. The mechanism of EHD-enhanced heat transfer in double-wall-heated channels is different from that in single-wall-heated counterparts. An advantageous electrode arrangement in which one electrode impinges on the top wall and the other one impinges on the bottom wall is proposed. Furthermore, partially double-wall-grounded pattern is also employed. Based on the above superior design, the heat transfer enhancement level by EHD is dramatically improved, which is enhanced by 166.4% for the top wall and 242.7% for the bottom wall compared with those without EHD effect. The mechanism can be attributed to eliminating the so-called negative effect of “extra electric body force” in the channel and removing the “barrier effect” between the bulk flow and the corona jet-induced vortex. Besides, the effects of longitudinal position of the electrode pair (d) and the gap between two electrodes (l) are also examined. It is found that heat transfer enhancements for the top and the bottom wall both decrease with d. It is also observed that as l increases, enhanced level for the top wall is firstly increased and then gradually reduced, while it is monotonically enhanced for the bottom wall. The effects of d and l are determined by combination of the negative effect of “extra electric body force” in the channel and the vortex temperature.

Numerical Analysis of Unsteady Conjugate Natural Convection of Hybrid Water-Based Nanofluid in a Semicircular Cavity

Unsteady conjugate natural convection in a semicircular cavity with a solid shell of finite thickness filled with a hybrid water-based suspension of Al2O3 and Cu nanoparticles (hybrid nanofluid) has been analyzed numerically. The governing equations for this investigation are formulated in terms of the dimensionless stream function, vorticity, and temperature and have been solved by the finite difference method of the second-order accuracy. The effects of the dimensionless time, Rayleigh number, thermal conductivity ratio, and the nanoparticles volume fraction on the flow patterns and heat transfer have been studied. The obtained results have revealed essential heat transfer enhancement at solid–fluid interface with addition of nanoparticles. In addition, a comparison of the heat transfer enhancement level due to the suspension of various nanoparticles materials (Al2O3 and Cu) in water as regular nanofluids (Al2O3/water and Cu/water) and as a hybrid Al2O3–Cu/water nanofluid is reported.

Experimental study on bubble dynamics and wall heat transfer arising from a single nucleation site at subcooled flow boiling conditions – Part 2: Data analysis on sliding bubble characteristics and associated wall heat transfer

This second of two companion papers presents an analysis of sliding bubble and wall heat transfer parameters measured during subcooled boiling in a square, vertical, upward flow channel. Bubbles were generated only from a single nucleation site for better observation of both the sliding bubble characteristics and their impact on wall heat transfer through optical measurement techniques. Specific interests include: (i) bubbles departure and subsequent growth while sliding, (ii) bubbles release frequency, (iii) coalescence of sliding bubbles, (iv) sliding bubbles velocity, (v) bubbles size distribution and (vi) wall heat transfer influenced by sliding bubbles.The results showed that sliding bubbles involve two distinct growth behaviors: (i) at low mass fluxes, sliding bubbles grew fast near the nucleation site, subsequently shrank, and then grew again, (ii) as mass flux increased, however, sliding bubbles grew more steadily. The bubbles originating from the single nucleation site coalesced frequently while sliding, which showed close relation with bubbles release frequency. The sliding bubble velocity near the nucleation site consistently decreased by increasing mass flux, while the observation often became reversed as the bubbles slid downstream due to the effect of interfacial drag. The sliding bubbles moved faster than the local liquid (i.e., ur<0) at low mass flux conditions, but it became reversed as the mass flux increased. The size distribution of sliding bubbles followed Gaussian distribution well both near and far from the nucleation site. The standard deviation of bubble size varied insignificantly through sliding compared to the changes in mean bubble size.Lastly, the sliding bubbles enhanced the wall heat transfer and the effect became more noticeable as inlet subcooling/mass flux decreased or wall heat flux increased. In particular, the sliding bubble characteristics such as bubble growth behavior observed near the nucleation site played a dominant role in determining the ultimate level of wall heat transfer enhancement within the test channel.

Heat Transfer and Pressure Loss Measurements of Matrix Cooling Geometries for Gas Turbine Airfoils

Matrix cooling systems are relatively unknown among gas turbines manufacturers of the western world. In comparison to conventional turbulated serpentines or pin–fin geometries, a lattice–matrix structure can potentially provide higher heat transfer enhancement levels with similar overall pressure losses. This experimental investigation provides heat transfer distribution and pressure drop of four different lattice–matrix geometries with crossing angle of 45 deg between ribs. The four geometries are characterized by two different values of rib height, which span from a possible application in the midchord region up to the trailing edge region of a gas turbine airfoil. For each rib height, two different configurations have been studied: one having four entry channels and lower rib thickness (open area 84.5%), one having six entry channels and higher rib thickness (open area 53.5%). Experiments were performed varying the Reynolds number Res, based on the inlet subchannel hydraulic diameter, from 2000 to 12,000. Heat transfer coefficients (HTCs) were measured using steady state tests and applying a regional average method; test models have been divided into 20 stainless steel elements in order to have a Biot number similitude with real conditions. Elements are 10 per side, five in the main flow direction, and two in the tangential one. Metal temperature was measured with embedded thermocouples, and 20 thin-foil heaters were used to provide a constant heat flux during each test. A specific data reduction procedure has been developed so as to take into account the fin effectiveness and the increased heat transfer surface area provided by the ribs. Pressure drops were also evaluated measuring pressure along the test models. Uniform streamwise distributions of Nusselt number Nus have been obtained for each Reynolds number. Measurements show that the heat transfer enhancement level Nus/Nu0 decreases with Reynolds but is always higher than 2. Results have been compared with previous literature data on similar geometries and show a good agreement.

Numerical simulation of power-law fluids flow and heat transfer in a parallel-plate channel with transverse rectangular cavities

It is aimed to study forced convection heat transfer for non-Newtonian power-law fluids in a parallel-plate channel with transverse rectangular cavities numerically. A finite volume based computation was performed using power-law discretization scheme and SIMPLE algorithm. The flow is assumed to be two-dimensional, incompressible, laminar and steady. Complex flow patterns such as deflection and re-circulation caused by the variation of cross-section area along the stream wise direction have been studied. Also, temperature distribution influenced by these perturbations has been discussed. In particular, the effects of Reynolds number (50≤Re≤350), power law index (0.5≤n≤2) and aspect ratio of channel cavities (A.R=0.25, 0.5) on heat transfer characteristics have been explored for channels of single and double cavity configuration. In all examined cases, varying levels of heat transfer enhancement were observed. The constant wall temperature condition has been applied.