Solar Air Heater Channel Research Articles

Effect of atmospheric wind on natural convection based solar air heaters

A detailed numerical investigation is carried out in this work to understand the effect of atmospheric wind on the heat transfer in natural convection based solar air heaters. A 3D inclined rectangular channel has been considered for the numerical simulations. The top wall of the channel is maintained at constant heat flux condition, and other walls of the channel are assumed to be adiabatic in all simulations. The effect of surface radiation between the channel walls and channel walls to the atmosphere is also considered. All the inner surface of the walls of the channel are assumed to be black and the emissivity (ε) of the plates are taken to be 0.95. Numerical simulations are carried out by varying the values of heat flux, q” = 250, 500 and 750 W/m2, velocity of the atmospheric wind Vwind = 0.0, 0.2, 0.6, and 1.0 m/s, and the inclination angle of the channel θ = 15°, 30°, 45°, and 60°. In the parametric study, various directions of free stream flow are also considered such as cross flow over the channel, in the main flow direction of the channel and the diagonal to the channel. In this paper, it is found that external wind imparts significant effects on the heat transfer and flow pattern of the air inside the solar air heater channel.

A performance and technoeconomic study of different geometrical designs of compact single-pass cross-matrix solar air collector with square-tube absorbers

This manuscript presents a performance study on a forced convection single-pass solar air heater channel with compact cross-matrix absorber (CMA) incorporating metal hollow square-tube absorbers. Four different geometries of CMA (Type I, II, III and IV) were investigated experimentally to evaluate their efficiency, pressure drops and heat transfer parameters. The experiments were conducted with uniform heat flux (indoor) and outdoor solar radiation as heat source. The air mass flow rates used were between 0.0142 kg/s and 0.0360 kg/s. Techno-economic feasibility studies were conducted using cost-benefit ratio (AC/AEG) method. Thermal efficiency of the CMA obtained by Type I with 76%, being the highest. CMA Type I also exhibited the highest temperature elevation than other configurations with 15.3 °C and thermal capacity of 38.7 kJ. Maximum pressure drop obtained was 1.33 Pa in turbulent condition with Reynolds number of 50,794. Type I has the advantage of high performance CMA and has comparatively lower cost-benefit ratio (AC/AEG) of 0.15 RM/kWh than other type of thermal absorbers.

Thermal characteristic in solar air heater fitted with plate baffles and heating corrugated surface

This paper presents a numerical investigation of heat transfer and friction factor characteristics in a solar air heater channel fitted with upper corrugated surface/wall and baffle series placed on a lower wall along the length of the channel. The corrugated surface/wall characterized by heating corrugated surface ratio (HCSR) varied from 0 to 0.5. The baffle series defined by blockage ratio (BR) fixed in 0.5. The fluid flow and heat transfer behaviors are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 8000 to 20000. The computations are based on the finite volume method, and the SIMPLE algorithm has been implemented. The present results show that the heat transfer rate and friction factor increase with the raise of Reynolds number. The best thermal performance factor observed at HCSR=0.5 tends to 2.7 at highest Reynolds number.

Heat transfer augmentation in a solar air heater channel with combined winglets and wavy grooves on absorber plate

An investigation on heat transfer characteristics in a solar air heater channel using wavy grooves incorporated with pairs of trapezoidal-winglets (TW) placed on the absorber plate has been experimentally conducted. Air as the test fluid entered the test section having a constant heat-flux on the absorber plate while the Reynolds numbers obtained by controlling the airflow rate is in a range of 4500–22,000. The TW characteristics include three relative winglet-pitches (PR) and five relative winglet height or blockage ratios (BR) at a single attack angle of 45° whereas the wavy rectangular-groove parameters are three relative groove-pitch lengths (PR) similar to the TW case but at a fixed width and height. The investigation shows that the TW together with the groove provides the substantial increase in heat transfer over the smooth channel. The TW alone gives much higher heat transfer but the groove yields considerably lower pressure drop. The combined groove and TW devices at a given BR, perform the highest heat transfer and friction factor at smaller PR and also provides considerably higher thermal performance than the single device acting alone. At PR=1, the compound device with BR=0.28 offers the highest heat transfer and friction factor while the one with BR=0.24 gives the maximum thermal performance.

Numerical Study of Heat Transfer Enhancement in Solar Air Heater Duct Fitted with Delta Winglets

The paper brings Numerical study of thermal performance of solar air heater channel in which delta winglet type vortex generators with and without holes punched on it are attached. Delta winglet type vortex generators having holes punched onto it are fitted in an array having 5 pair in one row, having 3 such rows starting at the entry of test section. With pitch of 60mm.Deltawinglet pair have an attack angle of 30 degrees, with height of winglet equal to half of duct height The study is carried out for Reynolds’s No in the range of 5000 to 25000 by changing airflow rate.in the test section with its upper channel wall provided with a uniform heat flux. Results are then compared with that of duct mounted with delta winglet without holes. Thermal performance is evaluated by analyzing both friction factor and Nussult’s number using R L Webb’s correlation for surfaces with roughness. Numerical simulation is done using Ansys fluent software. Analysis findings says that use of winglets brings enhancement in the heat transfer and thus increases the thermal performance. Arrangement of winglets also influences the performance. Improvements in design and performance can be brought by using winglets with different attack angles and span length.

Thermal Hydraulic Performance in a Solar Air Heater Channel with Multi V-Type Perforated Baffles

This article presents heat transfer and fluid flow characteristics in a solar air heater (SAH) channel with multi V-type perforated baffles. The flow passage has an aspect ratio of 10. The relative baffle height, relative pitch, relative baffle hole position, flow attack angle, and baffle open area ratio are 0.6, 8.0, 0.42, 60°, and 12%, respectively. The Reynolds numbers considered in the study was in the range of 3000–10,000. The re-normalization group (RNG) k-ε turbulence model has been used for numerical analysis, and the optimum relative baffle width has been investigated considering relative baffle widths of 1.0–7.0.The numerical results are in good agreement with the experimental data for the range considered in the study. Multi V-type perforated baffles are shown to have better thermal performance as compared to other baffle shapes in a rectangular passage. The overall thermal hydraulic performance shows the maximum value at the relative baffle width of 5.0.

Numerical studies on the effect of delta-shaped obstacles’ spacing on the heat transfer and pressure drop in v-corrugated channel of solar air heater

Solar air heater (SAH) is simple in construction compared to solar water heater. Yet, it is very useful for drying or space heating. Unfortunately, the convective heat transfer between the absorber plate and the air inside the solar air heater is rather low. Some researchers reported that obstacles are able to enhance the heat transfer in a flat plate solar air collector and others found that a v-corrugated absorber plate gives better heat transfer than a flat plate. Only a few research combines these two in a SAH. This paper will describe the combination from other point of view, i.e. the spacing between obstacles. Its spacing possibly will effect the heat transfer and pressure drop of the air flowing across the channel.The first step in numerical study is generating mesh or grid of the air flow inside a v-corrugated channel which was blocked by some delta-shaped obstacles. The mesh was designed three-dimension and not uniform. The mesh are made finer for area near obstacles and walls both for upper and bottom, and then gradually coarser. Grid independency is the next step to be conducted. When the mesh is already independent, the numerical study begins. To validate the numerical model, an indoor experiment was conducted. Turbulent model used was Shear Stress Transport K-ω (SSTK-ω) standard. Having a valid numerical model, the spacing between obstacles was studied numerically. Ratio spacing to height, S/H of obstacles investigated were 0.5; 1; 1.5; and 2.From numerical studies in a v-corrugated duct, it is found that backflow between obstacles and high velocity in the gap between obstacles and absorber plate causes the flow became more turbulent and enhanced the convection heat transfer between the air and the absorber plate. Obstacles placed in a small spacing will increase Nusselt number (convection heat transfer) and friction factor (pressure drop). The Nusselt number enhanced from 27.2 when no obstacle used to 94.2 when obstacles inserted with S/H=0.5. The Nusselt enhanced 3.46 times. The friction factor will increase from 0.0316 at no obstacle to 0.628 at ratio S/H=0.5. The friction factor increased 19.9 times. Efficiency, Nusselt number, and friction factor are decreasing as ratio S/H is increasing. When ratio S/H used is 1 instead of 0.5, Nusselt number enhancement decreased only 1.13%, but friction factor decreased 15.1%. So, sacrificing a small amount of Nusselt number but reducing a significant friction factor is advantageous. The optimal spacing ratio S/H of delta-shaped obstacles inserted in a v-corrugated SAH is one. In other words, the optimal spacing of obstacle equals to its height.

Thermal performance in solar air heater channel with combined wavy-groove and perforated-delta wing vortex generators

An experimental investigation on thermal performance improvement in a solar air heater channel with combined wavy-groove and delta-wing vortex generator (WVG) placed on the absorber plate having a uniform wall heat-flux is carried out. The Reynolds number based on the hydraulic diameter of the channel ranges from 4800 to 23,000. The effect of the combined groove and WVG on the heat transfer and pressure drop in the channel in terms of respective Nusselt number and friction factor is examined. Investigated parameters of the WVG mounted on the grooved absorber are three wing porosity area ratios (called porosity ratio, Ah/Aw = 0.031, 0.085 and 0.167) and four groove-wing distance to channel-height ratios (g/H = 0.4, 0.5, 0.75 and 1) at a single attack angle (α = 45°). The experimental result reveals that at g/H = 0.5, the smaller Ah/Aw provides the highest Nusselt number and friction factor around 6 and 30 times over the smooth channel, respectively, but the optimum thermal performance is at Ah/Aw = 0.085 and g/H = 0.5. The combined devices give the thermal performance augmentation at about 37.7–46.3% higher than the groove alone and also at about 1.5–12.5% above the combined groove and non-perforated WVG (without hole).

Field synergy analysis for turbulent heat transfer on ribs roughened solar air heater

Convective turbulent heat transfer field synergy equations for a solar air heater channel with rib turbulators are developed based on the extremum principle of heat transfer potential capacity dissipation. Field synergy equations can be used to increase the field synergy between the velocity vector and temperature gradient fields over the entire heat flow domain to enhance the convective turbulent heat transfer. Solution of the field synergy equations gives the optimal flow field of swirls flow under the ribs and in the cavity area (i.e. between rib to rib), which are followed by a gradual increase of Nusselt number up to following rib, having the best field synergy for a different viscous dissipation. Using optimal velocity patterns as a guide, the design of solid inclined, curved and vertical baffles can be introduced in actual applications, so as to generate the desired swirls flow and enhance the overall coverage of turbulent heat exchange. As illustrative examples, the local field synergy angle analyses for turbulent heat transfer in ribs roughened channel with/without solid baffles are presented. Results show that the solid curved baffles mounted near absorber plate has a best performance evaluation criterion of 0.99.

A general optimized geometry of angled ribs for enhancing the thermo-hydraulic behavior of a solar air heater channel – A Taguchi approach

The thermo-hydraulic behavior of the air flow through a solar air heater is numerically simulated and then optimized by use of Taguchi method. An innovative general geometry is introduced for the rib that variation in its parameters can generate triangular, trapezoidal and rectangular geometries simultaneously. A L16 (44) orthogonal array is used to optimize the geometry factors accounts for the maximum thermal performance of the ribbed channel. Thermal performance concept includes maximization of heat transfer coefficient and minimization of friction factor. Maximization of thermal enhancement factor is taken as the criteria of optimization. At a constant flow Reynolds number of 10,000, rib relative pitch (P/H), rib relative height (e/H), rib relative tip width (a/H) and rib front projection (s) are employed as the design factors. Results show that rib pitch, rib height, rib tip width and rib front projection have the greatest influences on the thermo-hydraulic performance, respectively. A triangular rib geometry with rib height of 0.2H and P = 2H in which the rib front is normal to the flow direction (i.e. s = 0) is recognized as the optimum configuration.

Effects of rib size and arrangement on forced convective heat transfer in a solar air heater channel

The article presents an experimental investigation on turbulent heat transfer and friction loss behaviors of airflow through a constant heat-fluxed solar air heater channel fitted with rib turbulators. The experiment was conducted for the airflow rate in terms of Reynolds numbers based on the hydraulic diameter of the channel in a range of 5000–24,000. In the present work, a comparative study between square and thin ribs (90°-rib) with three rib arrangements, namely, one ribbed wall (or single rib), in-line and staggered ribs on two opposite walls was first introduced. The study shows a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The comparison made at a single rib pitch and height also revealed that the thin rib performs better than the corresponding square one. Among the three arrangements, the in-line rib array provides higher heat transfer and friction loss than the staggered and the single one. However, the staggered thin rib provides the highest thermal performance. With this reason, only the staggered thin ribs at four different relative heights (BR = 0.1, 0.2, 0.3 and 0.4) and three relative pitches (PR = 0.5, 0.75 and 1.33) are investigated further. It is found that the staggered rib at BR = 0.4 and PR = 0.5 yields the highest heat transfer and friction factor but the maximum thermal performance is at BR = 0.2 and PR = 0.75.

Heat transfer behaviors in a solar air heater channel with multiple V-baffle vortex generators

The article presents a study on heat transfer augmentation in a solar air heater channel fitted with multiple V-baffle vortex generators (BVG). During the test air was passed through the test channel under a uniform wall heat-flux of the absorber plate. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the channel hydraulic diameter ranging from 4000 to 21,000. The V-baffles are applied at a relative baffle height (in terms of blockage ratio, BR=b/H=0.25) and attack angle of 45° with respect to the main flow direction. The use of BVG in the channel is to generate multiple longitudinal vortex flows through the test channel to increase turbulence intensity and stronger mixing of fluid between the core and the near-wall flow. Influences of three different baffle-pitch to channel-height ratios (PR=P/H=0.5, 1 and 2) on heat transfer and pressure drop in terms of respective Nusselt number and friction factor (or energy loss for propelling air through the channel) are examined. Three BVG arrangements, namely, one BVG wall (or single BVG), in-line and staggered BVGs on two opposite walls are also investigated. The experimental result reveals that the smaller PR provides the highest heat transfer and friction factor for all BVGs. The in-line BVG yields higher heat transfer and friction loss than the staggered and the single BVG. However, the single BVG with PR=0.5 yields the highest thermal performance. To shed light of heat transfer mechanism, a numerical work is also conducted to investigate heat transfer and flow friction characteristics in the channel fitted with 45° BVGs and in comparison, the numerical results are in good agreement with experimental data.

Thermal Behaviors in a Solar Air Heater Channel with Arc-Shaped Baffle Turbulators

Influences of arc-shaped baffles turbulators (ASB) on thermal behaviors in a solar air heater channel are studied. In the experiment, the ASB with three pitch ratios (P/e) are placed on the absorber plate (bottom channel wall in this case) to generate stronger turbulence intensity in the channel. The distributions of temperature and local Nusselt number contours in the absorber plate fitted with ASB are observed with thermochromic liquid crystal (TLC) sheet. The results reveal that the heat transfer increases with the decrease in P/e due to longitudinal vortex flow effect. The heat transfer from employing the ASB at P/e = 4.0, 6.0 and 8.0 is higher than the smooth channel around 116%, 120% and 127%, respectively. This indicates that the ASB pitch spacing is optimal at 8 times baffle height in the present work.

Numerical Heat Transfer Investigation in Solar Air Heater Channel with Wavy-Baffles

A numerical study is conducted to investigate the turbulent periodic flow and heat transfer characteristics in a channel fitted with sinusoidal wavy-baffles placed on upper and lower walls. The finite volume method is introduced and implemented with the SIMPLE algorithm. The flow structure, friction factor and heat transfer characteristics for different wavy-baffle configurations are evaluated. According to numerical result, the maximum thermal performance is found to be 1.22–1.66 times to smooth channel corresponding to the baffle a/H=0.75 and b/H=0.10. Consequently, the application of wavy-baffles can be utilized effectively to enhance the thermal performance of solar air heater.

Heat Transfer Enhancement in a Solar Air Heater Channel with Discrete V-Baffles

This article presents the study of heat transfer enhancement in a uniform heat-fluxed channel fitted with discrete V-shaped baffles. The experiments are carried out by varying airflow rate for Reynolds number ranging from 4100 to 22,000. The V-baffles with relative height ratio, e/H = 0.15 and the attack angle, α = 45o, are mounted repeatedly on the upper plate only, similar to an absorber plate of solar air heater systems. The effects of four baffle-pitch to channel-height ratios (PR= 0.5, 1.0, 1.5 and 2.0) on heat transfer in terms of Nusselt number and pressure loss in the form of friction factor are experimentally investigated. The experimental results show that the use of the discrete V-baffles leads to a considerable increase in Nusselt number and friction factor in comparison to the smooth channel alone. The V-baffled channel with PR=0.5 provides the highest heat transfer, friction factor and thermal enhancement factor.

Experimental Investigation on Turbulent Convection in Solar Air Heater Channel Fitted with Delta Winglet Vortex Generator

The paper presents an experimental study on the heat transfer and flow friction characteristics in a solar air heater channel fitted with delta-winglet type vortex generators (DWs). The experiments are conducted by varying the airflow rate for Reynolds number in the range of 5000 to 24000 in the test section with a uniform heat-flux applied on the upper channel wall. Firstly, the DW pairs are mounted only at the entrance of the lower wall of the test channel (called DW-E) to create multiple vortex flows at the entry. The effect of two transverse pitches (RP = Pt/H = 1 and 2) at three attack angles (α = 30°, 45° and 60°) of the DW-E with its relative height, b/H = 0.5 (half height of channel) is examined. Secondly, the 30° DWs with three different relative heights (b/H = 0.3, 0.4 and 0.5) are placed on the upper wall only (absorber plate, called DW-A) of the test channel. The experimental result reveals that in the first case, the 60° DW-E at RP = 1 provides the highest heat transfer and friction factor while the 30° DW-E at RP = 1 performs overall better than the others. In the second case, the 30° DW-A at b/H = 0.5 yields the highest heat transfer and friction factor but the best thermal performance is found at b/H = 0.4.

Thermal performance of turbulent flow in a solar air heater channel with rib-groove turbulators

The paper presents an experimental study on turbulent flow and heat transfer characteristics in a solar air heater channel fitted with combined wavy-rib and groove turbulators. The experiments are performed by controlling the airflow rate to obtain Reynolds numbers in the range of 4000 to 21,000. To produce recirculation flow in the tested channel having a constant heat-flux on the upper wall only, the triangular wavy ribs are placed repeatedly on the tested grooved channel walls. Three test cases of different rib-pitch to channel-height ratios (PR=P/H=0.5, 1 and 2) with a single rib-to-channel height ratio (BR=b/H=0.25) are introduced in the present work. The wavy ribs are placed with the attack angle of 45° relative to main flow direction. There are three types of rib arrangements, namely, rib-groove on the upper wall only, inline rib-groove, and staggered rib-inline groove on two principal walls. The experimental result reveals that the combined rib-groove on both the upper and lower walls of the test channel provides the highest heat transfer rate and friction factor in comparison with the smooth channel with/without ribs. However, the ribbed-grooved upper wall at PR=0.5 yields the highest thermal performance. The combined rib-groove turbulator is found to be considerably higher thermal performance than the groove alone.

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles

The influence of baffle turbulators on heat transfer augmentation in a rectangular channel has been investigated experimentally and numerically. In the experiment, the baffles are placed in a zigzag shape (Z-shaped baffle) aligned in series on the isothermal-fluxed top wall, similar to the absorber plate of a solar air heater channel. The aim at using the Z-baffles is to create co-rotating vortex flows having a significant influence on the flow turbulence intensity leading to higher heat transfer enhancement in the tested channel. Effects of the Z-baffle height and pitch spacing length are examined to find the optimum thermal performance for the Reynolds number from 4400 to 20,400. The Z-baffles inclined to 45° relative to the main flow direction are characterized at three baffle- to channel-height ratios (e/H=0.1, 0.2 and 0.3) and baffle pitch ratios (P/H=1.5, 2 and 3). The experimental results show a significant effect of the presence of the Z-baffle on the heat transfer rate and friction loss over the smooth channel with no baffle. The Nusselt number, friction factor and thermal performance enhancement factor for the in-phase 45° Z-baffles are found to be considerably higher than those for the out-phase 45° Z-baffle at a similar operating condition. The in-phase 45° Z-baffle with larger e/H provides higher heat transfer and friction loss than the one with smaller e/H while the shorter pitch length yields the higher Nu, f and TEF than the larger one. The numerical work is also conducted to investigate the flow friction and heat transfer behaviors in the channel mounted with the 45° Z-baffles, and the numerical results are found in good agreement with experimental data.

Distributed nonstationary heat model of two-channel solar air heater

An algorithm for a distributed nonstationary heat model of a solar air heater (SAH) with two operating channels is presented. The model makes it possible to determine how the coolant temperature changes with time along the solar air heater channel by considering its main thermal and ambient parameters, as well as variations in efficiency. Examples of calculations are presented. It is shown that the time within which the mean-day efficiency of the solar air heater becomes stable is significantly higher than the time within which the coolant temperature reaches stable values. The model can be used for investigation of the performances of solar water-heating collectors.

Thermal behavior in solar air heater channel fitted with combined rib and delta-winglet

Effects of combined ribs and delta-winglet type vortex generators (DWs) on forced convection heat transfer and friction loss behaviors for turbulent airflow through a solar air heater channel are experimentally investigated in the present work. Measurements are carried out in the rectangular channel of aspect ratio, AR = 10 and height, H = 30 mm. The flow rate is presented in the form of Reynolds numbers based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The cross-section shape of the rib placed on the absorber plate to create a reverse-flow is an isosceles triangle with a single rib height, e/H = 0.2 and rib pitch, P l /H = 1.33. Ten pairs of the DW with its height, b/H = 0.4; transverse pitch, P t / H = 1 and three attack angles ( α) of 60°, 45° and 30° are introduced and mounted on the lower plate entrance of the tested channel to generate longitudinal vortex flows. The experimental results show that the Nusselt number and friction factor values for combined rib and DW are found to be much higher than those for the rib/DW alone. The larger attack angle of the DW leads to higher heat transfer and friction loss than the lower one. In common with the rib, the DW pointing upstream (PU-DW) is found to give higher heat transfer rate and friction loss than the DW pointing downstream (PD-DW) at a similar operating condition. In comparison, the largest attack angle ( α = 60°) of the PU-DW yields the highest increase in both the Nusselt number and friction factor while the lowest attack angle of the PD-DW provides the best thermal performance.