A flat-plate solar collector (FPSC) using multi-walled carbon nanotubes (MWCNTs) was numerically studied. Multi-walled carbon nanotubes (MWCNTs) with outside diameters of (< 8 nm) and 0.1wt.% were utilized. A three-dimensional model was built and solved via ANSYS software and the inlet parameters as 1000 W/m2, inlet temperature of 30°C and the volume flow rates in the range of 0.2-0.8 kg/min. Using DW decreased the temperature of absorber by 0.840%, 1.437%, 1.909%, 2.308%, 2.616% and 2.869% for the varied flow rates. Relative to DW, the temperature of absorber decreased by 0.874%, 0.804%, 0.756%, 0.717%, 0.685%, 0.655% and 0.633% at the same flow rate ranges. Meanwhile, the thermal efficiency of MWCNTs nanofluid was increased by 6.080%, 6.322%, 6.311%, 6.337%, 6.450% and 6.857% for volume flow rate of 0.2-0.8 kg/min

Alawi, Omer A.

Mohamed Kamar, Haslinda

Universiti Teknologi Malaysia Institutional Repository

 Journal of Advanced Research in Micro and Nano Engineering 2, Issue 1 (2020) 12-21  12   Journal of Advanced Research in Micro and Nano Engineering  Journal homepage: www.akademiabaru.com/armne.html ISSN: 2756-8210  Performance of Solar Thermal Collector Using Multi-Walled Carbon Nanotubes: Simulation Study   Omer A. Alawi1,*, Haslinda Mohamed Kamar1,*   1 Department of Thermofluids, School of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia   ABSTRACT A flat-plate solar collector (FPSC) using multi-walled carbon nanotubes (MWCNTs) was numerically studied. Multi-walled carbon nanotubes (MWCNTs) with outside diameters of (< 8 nm) and 0.1wt.% were utilized. A three-dimensional model was built and solved via ANSYS software and the inlet parameters as 1000 W/m2, inlet temperature of 30°C and the volume flow rates in the range of 0.2-0.8 kg/min. Using DW decreased the temperature of absorber by 0.840%, 1.437%, 1.909%, 2.308%, 2.616% and 2.869% for the varied flow rates. Relative to DW, the temperature of absorber decreased by 0.874%, 0.804%, 0.756%, 0.717%, 0.685%, 0.655% and 0.633% at the same flow rate ranges. Meanwhile, the thermal efficiency of MWCNTs nanofluid was increased by 6.080%, 6.322%, 6.311%, 6.337%, 6.450% and 6.857% for volume flow rate of 0.2-0.8 kg/min. Keywords:  Flat-plate solar collector; Thermal efficiency; Carbon-based nanofluid; MWCNTs nanoparticles Copyright © 2020 PENERBIT AKADEMIA BARU - All rights reserved  1. Introduction  Flat plate solar collectors (FPSCs) are basic devices used to deliver hot water required for usual practices. FPSC can be installed above the housing or public buildings since its mechanism is simple. The daylight passes over one or two glass layer (s) and strikes to the thin flat plate made of aluminum which with a special coating to absorb more energy. Then, the heat is transferred to the working fluid flowing through riser pipes attached to the flat plate absorber [1][2].   However, considering the low cost of maintaining flat plate collector systems and no need for sun monitoring, their low thermal performance is one of the researchers' biggest challenges. However, investigators have come up with many findings to resolve this key deficiency, such as suspension nanoparticles (1nm-1 µm) made of metallic or non-metallic within the base fluids or alteration of the geometry of the absorber to obtain an effective design  [3],[4],[5]. Carbon nanostructures such as (SWCNTs, MWCNTs, GNPs, GO & Gr) were used as  HTFs  rather than basic fluids inside the FPSCs [6], [7], [8], [9]. Said et al. [6] and [10] tested SWCNTs-H2O nanofluid  through theoretical and experimental studies in terms of heat transport, pressure drop, and exergy                                                           * Corresponding author. E-mail address: omeralawi@utm.my * Corresponding author. E-mail address: haslinda@mail.fkm.utm.my   Open Access Journal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  13  efficiency of the solar  absorber. The energy and exergy performance reached about 95% and 26.25%, respectively when using 3 wt.%-SWCNT-H2O at 0.5 kg/min. Also, the solar energy efficiency was increased by 28.6% when using 0.2 wt.%-MWCNTs-H2O at 2 kg/min [11].Vakili et al. [8] increased the energy efficiency of solar collector to 13.5%, 19.7% and 23.2% for 0.0005wt.%, 0.001wt.% and 0.005wt.% of GNPs-H2O, respectively at 0.9 kg/min. Ahmadi et al. [12] showed that, the thermal performance of solar collector was enhanced by 18.9% with 0.02wt.%-Gr-H2O at 0.9 kg/min through theoretical and experimental investigations. Moreover, experimental findings showed that the highest energy efficiency was gained after using 0.1wt.%-CGNP-H2O nanofluid with 0.026 kg/s.m2, which was about 18.2% higher than H2O. Triple nanofluid (MWCNTs/GNPs/h-BN) was tested as a heat transfer fluid under three different volume flow rates such as (2 L/min, 3 L/min, and 4 L/min) [14]. Therefore, the hybrid nanofluid with 4 L/min exhibited the maximum thermal-efficient solar collector up to 85%. Sarsam and his team in two different studies [15],[16] examined (TEA-GNPs) and (Ala-MWCNTs) inside indoor FPSC. Relative to DW, the thermal efficiency improved up to 10.53% for  (0.1wt.%-TEA-GNPs) nanofluid with specific surface area (SSA) of 750 m2/g. While, the FPSC’s effectiveness improved up to 9.55% for 0.1-wt.% Ala-MWCNTs < 8 nm at 1.4 kg/min, compared with DW. More research is needed to understand the usage of carbon based nanofluids as HTFs inside solar collectors. In this paper, a 3D numerical model was solved under conjugated laminar mixed convection heat transfer using MWCNTs-H2O with outside diameters of (< 8 nm). Different parameters were taken into consideration such as; the mass fraction of 0.1wt.%, the heat flux of 1000 W/m2, the inlet temperature of 30°C and the volume flow rates in the range of 0.2-0.8 kg/min.    2. Methodology  Figure 1(a) demonstrates the current model of 3D-FPSC working under conjugated laminar mixed convection and the solar radiation was applied as a constant heat flux at the absorber surface. The basic model contains a thin absorber plate made of aluminum and a riser pipe made of copper. The detailed dimensions are summarized in Table 1 as; the flat plate length (L), flat plate width (w), flat plate thickness (h), pipe thickness (d) and pipe inner diameter (D). The CFD domain is also subdivided into many simple small cells with the purpose of having better control over grid sizes and enhance the meshing efficiency. As shown in Fig. 1(b), the feature of inflation is selected near the walls of the riser pipe.   (a) Journal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  14   (b) Fig. 1. (a) A schematic diagram of FPSC, (b) Grid domain Table 1 Specifications of geometrical parameters of FPSC  Dimension Value Dimension Value L 914.4 mm D 10.5 mm w 128 mm h 2 mm d 0.5 mm θ 30°  3. Mathematical Modelling  Conjugated laminar mixed convection of DW and MWCNTs/H2O was examined using a 3D-FPSC with tilt angle of 30°. The fully developed condition is assumed at the inlet boundary condition, the working fluids have uniform temperature The thermophysical properties of DW and MWCNTs/H2O nanofluid are taken from the literature review at the inlet condition of 30°C [16]. As mentioned earlier, the flow is considered to be steady-state condition, Newtonian, and laminar. The flat absorber is heated by a constant wall heat flux while non-slip and adiabatic boundary conditions are set for the lateral absorber plate walls and lower outer riser pipe. Gravitational force is applied in the normal direction of (y-axis) with a value of (-9.81 m/s2). Furthermore, body forces, solar radiation and compressibility are neglected while DW and MWCNTs nanoparticles are assumed as a single-phase in thermal equilibrium with zero relative velocity. The continuity, momentum  and energy governing equations  are as follows [17], [18]:  𝜕𝜕𝑥𝑖(𝜌𝜇𝑖) = 0              (1)  𝜕𝜕𝑥𝑗(𝜌𝜇𝑖𝜇𝑗) =𝜕𝑝𝜕𝑥𝑖+𝜕𝑝𝜕𝑥𝑗(𝑣 (𝜕𝜇𝑖𝜕𝑥𝑗+𝜕𝜇𝑗𝜕𝑥𝑖) − ?̀?𝑖?̀?𝑗)         (2)  𝜕𝜌𝑐𝑝𝜇𝑗𝑇𝜕𝑥𝑖=𝜕𝜕𝑥𝑗(𝑘𝜕𝑇𝜕𝑥𝑗− 𝜌𝑐𝑝𝜇𝑗𝑇̅̅ ̅̅ ̅) + 𝑆𝑇         (3)  where 𝑖 = 1,2,3, 𝑢𝑖 = (𝑢, 𝑣, 𝑤) represent velocity vectors. The boundary conditions (BCs)  of the problem can be explained as follows [19],[20],[21]: Inlet pipe section: 𝑢 = 𝑣 = 0,                (4) Journal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  15   𝑤 = 𝑈𝑖𝑛,𝑇 = 𝑇𝑖𝑛                  (5)  Upper surface of the absorber:  𝑢 = 𝑣 = 𝑤 = 0                 (6)  𝑞”𝑤 = 𝐼(𝜆𝑘) − ℎ(𝑇𝑐𝑜𝑙 − 𝑇𝑎𝑚𝑏)             (7)  Adiabatic walls:  𝑢 = 𝑣 = 𝑤 = 0                  (8)  𝜕𝑇𝜕𝑦= 0                (9)  Outlet pipe section:  𝜕𝑢𝜕𝑧=𝜕𝑣𝜕𝑧=𝜕𝑤𝜕𝑧= 0                          (10)  𝜕𝑇𝜕𝑧= 0                          (11)  Solid-liquid interfaces:  𝑘𝜕𝑇𝜕𝑥= 𝑘𝑠𝜕𝑇𝑠𝜕𝑥                           (12)  The solar collector efficiency can be written as by:  𝜂 =𝑄𝑢𝐴𝑎𝐼=?̇?𝑐𝑝(𝑇𝑜𝑢𝑡−𝑇𝑖𝑛)𝐴𝑎𝐼                          (13)   4. Results and discussion  Figure 2 presents the values of flat plate absorber temperature using DW and MWCNTs/H2O at different volume flow rates. It can be seen from the presented data; the value of absorber temperature decreases as volume flow rates increases for both cases (water and nanofluid).  The temperature of the absorber using water was also higher than that of nanofluid, which is characteristic of the convective mode of heat transfer under constant heat flux [15].  Figures (3-4) show the results of gain energy and thermal efficiency using water and nanofluids for varied flow rates. It is evident from this that the efficiency of the collector improves as the flow rate increases, which can be due to the decreased flat plate temperature (Fig. 3), resulting in lower collector heat losses, i.e., improved collector efficiency.  Using DW decreased the temperature of absorber by 0.840%, 1.437%, 1.909%, 2.308%, 2.616% and 2.869% for the varied flow rates. Relative to DW, the temperature of absorber decreased by 0.874%, 0.804%, 0.756%, 0.717%, 0.685%, 0.655% and 0.633% at the same flow rate ranges. Meanwhile, the thermal efficiency of MWCNTs nanofluid was increased by 6.080%, 6.322%, 6.311%, 6.337%, 6.450% and 6.857% for volume flow rate of 0.2-0.8 kg/min.  Journal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  16  Figure 5 discusses the effect of volume flow rates on the thermal field for both working fluids (water and nanofluid) in terms of temperature contours. The surface temperature of flat plate using DW decreases as the volume flow rate increases. Meanwhile, the surface temperature of flat plate using DW is higher than the nanofluid. It can be firmly accepted that nanofluid is the most powerful alternative working fluid that can be used instead of traditional working fluids for higher energy efficiency in solar collectors or any other comparable thermal equipment or engineering process.     Fig. 2. Temperature of flat plate absorber using water and nanofluid at different flow rates   Fig. 3. Qain energy for water and MWCNTs/H2O at different volume flow rates 3103153203253303353400 0.2 0.4 0.6 0.8 1Temperature (K)Z (m)DW_0.2 kg/min DW_0.3 kg/min DW_0.4 kg/minDW_0.5 kg/min DW_0.6 kg/min DW_0.7 kg/minDW_0.8 kg/min NF_0.2 kg/min NF_0.3 kg/minNF_0.4 kg/min NF_0.5 kg/min NF_0.6 kg/minNF_0.7 kg/min NF_0.8 kg/min54.27663.42169.43275.84580.92984.73990.85257.57667.43173.81480.69286.05790.20497.0820.2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8Qain energy (W)Volume flow rate (kg/min)DW NanofluidJournal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  17   Fig. 4. Thermal efficiency for water and MWCNTs/H2O at different volume flow rates    DW_0.2 kg/min  NF_0.2 kg/min  DW_0.3 kg/min  NF_0.3 kg/min 46.37254.18659.32264.80169.14472.39977.62349.19257.61263.06668.94273.52677.06982.9460.2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8Thermal efficiency (%)Volume flow rate (kg/min)DW NanofluidJournal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  18   DW_0.4 kg/min  NF_0.4 kg/min  DW_0.5 kg/min  NF_0.5 kg/min  DW_0.6 kg/min  NF_0.6 kg/min  DW_0.7 kg/min  NF_0.7 kg/min Journal of Advanced Research in Micro and Nano Engineering  Volume 2, Issue 1 (2020) 12-21  19   DW_0.8 kg/min  NF_0.8 kg/min  Fig. 5. Temperature contours of flat plate absorber using DW and nanofluid at different volume flow rates   5. Conclusions   Simulation studies were performed using 3D model for flat plate solar collector to examine the thermal performance of MWNCTs/H2O nanofluid. The thermophysical properties of 0.1wt.% MWCNTs were taken from the literature review. Forced convective flow was taken into account under volume flow rate of 0.2-0.8 kg/min. Meanwhile, the inlet temperature for DW and MWNCTs/H2O nanofluid was fixed at 30°C with a constant heat flux of 1000 W/m2 was applied at the absorber plate as input heat. Using DW decreased the temperature of absorber by 0.840%, 1.437%, 1.909%, 2.308%, 2.616% and 2.869% for the varied flow rates. Relative to DW, the temperature of absorber decreased by 0.874%, 0.804%, 0.756%, 0.717%, 0.685%, 0.655% and 0.633% at the same flow rate ranges. Meanwhile, the thermal efficiency of MWCNTs nanofluid was increased by 6.080%, 6.322%, 6.311%, 6.337%, 6.450% and 6.857% for volume flow rate of 0.2-0.8 kg/min.  Acknowledgments This work was supported by Universiti Teknologi Malaysia (UTM), grant from Research Management Centre (RMC) (04E76).   References  [1] Pandey, Krishna Murari, and Rajesh Chaurasiya. 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Performance of solar thermal collector using multi-walled carbon nanotubes: simulation study

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Performance of solar thermal collector using multi-walled carbon nanotubes: simulation study

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