Experimental Study on the Enhancement of Pool Boiling Heat Transfer Characteristics of Water-Based Nanofluids with Graphene Nanoplatelets on Nichrome Wire

Abstract

The present study aims to experimentally investigate pool boiling heat transfer characteristics, such as critical heat flux (CHF) and boiling heat transfer coefficient (BHTC), of pure distilled water (d-H₂O) and functionalised graphene nanoplatelet (f-GnPs)–d-H₂O nanofluids using a nichrome (Ni-Cr) test wire as the heating element. The distilled water (dH₂O) and GnP (5–10 nm and 15 µm, Cheap Tubes, USA) were chosen as the base fluid and nanomaterial, respectively. The GnP was chemically functionalized and dispersed in dH₂O using a probe sonicator. The nanofluids were characterized by measuring the zeta potential distribution and pH to ensure stability on day 1 and day 10 following preparation. The results show that the zeta potential values range from −31.6 mV to −30.6 mV, while the pH values range from 7.076 to 7.021 on day 1 and day 10, respectively. The novelty of the present study lies in the use of f-GnPs with a controlled size and stable nanofluid, confirmed through zeta potential and pH analysis, to determine the heat transfer behaviour of a Ni-Cr test wire under pool boiling conditions. The pool boiling heat transfer characteristics, such as CHF and BHTC, were observed using the fabricated pool boiling heat transfer test facility. Initially, the dH₂O and f-GnP–dH₂O nanofluids were separately placed in a glass container and heated using a pre-heater to reach their saturation point of 100 °C. The electrical energy was gradually increased until it reached the critical point of the Ni-Cr test wire, i.e., the burnout point, at which it became reddish-yellow hot. The CHF and BHTC were predicted from the experimental outputs of voltage and current. The results showed an enhancement of ~15% in the CHF at 0.1 vol% of f-GnPs. The present study offers a method for enhancing two-phase flow characteristics for heat pipe applications.

1. Introduction

In the current decade, the energy crisis has drawn significant attention to the need for efficient energy saving in compact systems that remove heat. Energy-efficient devices face high demand and require efficient thermal management techniques for compact systems, especially given the energy crisis scenario in various industries. The boiling phenomenon is an advanced heat transfer process widely used in multiple two-phase heat transfer systems such as heat pipes, evaporators, and boilers. The CHF was observed to be lower in conventional two-phase flow systems, particularly due to the rapid phase change of liquids [1,2,3,4,5]. Researchers worldwide are conducting studies using various techniques to increase the CHF, particularly in the nucleate boiling regime, due to delays in reaching the transition boiling region of liquids.

There are two ways to increase the CHF: using an effective surface and heat transfer fluids. Modification of the actual surface to an effective surface could affect the compactness of the entire system; this has been the focus of researchers in enhancing the properties of heat transfer fluids (base fluids) with the addition of nanomaterials referred to as nanofluids. Conventional base fluids, namely water, ethanol, engine oil, and EG, which have lower heat conduction, result in less CHF. The addition of higher conductive nanomaterials to those base fluids could enhance heat conduction and boiling heat transfer characteristics [6,7,8,9,10,11,12].

Experimental investigations have revealed that base fluids dispersed with various nanomaterials significantly enhance the CHF. Many researchers have investigated pool boiling heat transfer characteristics using metal and metal oxide nanomaterials dispersed with H₂O. Vassallo et al. [13] investigated pool boiling heat transfer using a horizontal wire submerged in a Si-H₂O nanofluid. They reported a 60% increase in CHF with Si-H₂O nanofluids at 0.5 vol% with respect to pure H₂O. Bang and Chang [14] examined variations in CHF of Al₂O₃-H₂O nanofluids at various vol% of Al₂O₃-H₂O, namely 0.5%, 1%, 2%, and 4%. The 32% increment in CHF was seen at 4 vol% of Al₂O₃ in H₂O. Kathiravan et al. [15] studied the use of CHF of Cu-dH₂O nanofluids at various wt% of Cu, such as 0.25%, 0.5%, and 1.0%, with SDS surfactant. The CHF was reported to be enhanced by up to 60% at 1.0 wt%. Reddy and Venkatachalapathy [16] studied the enhancement of CHF through dispersal of two different nanoparticles, viz., Al₂O₃ with CuO, at various vol% of 0.01 to 0.1% in DI-H₂O. The observed enhancement in critical heat flux was 49.84% for Al₂O₃ with CuO nanomaterials, respectively, at 0.1 vol%. Milanova and Kumar [17], Song et al. [18], and Ajeeb and Murshed [19] had similar observations.

In the current decade, carbon nanomaterials, namely CNT, GO, and GnP, have gained more attention for enhancing boiling heat transfer characteristics in H₂O due to their significant thermal properties and lower density. Kamatchi and Venkatachalapathy [20] reviewed various boiling heat transfer studies on the enhancement of CHF using different types of nanofluids. The authors reported a superior enhancement of CHF from the dispersion of carbon-based nanomaterials, especially GnP, due to their significant thermal properties and shape. Akbari et al. [21] focused on the effects of GnP nanomaterials by dispersing them in H₂O and reported an enhancement of up to 72% in CHF at a mass percentage of 0.1%. Akbari et al. [22] studied the effects of carbon-based nanomaterials, namely GnP, PEG-functionalized GnP, and MWCNT, dispersed with DI-H₂O at various wt% concentrations (0.01%, 0.05%, and 0.1%), on CHF. The results show that the CHF of GNP−PEG/DI-H₂O nanofluids was enhanced by 72% for 0.1 wt%, while there was a 55% enhancement for the GnP/DI-H₂O nanofluid compared to CNT/DI-H₂O nanofluids. Sudhir and Deepak [23] experimentally studied two-phase heat transfer by heating a Cu test surface using water-based nanofluids with various nanomaterials, namely GnP, GO, and r-GO, at volumes of 0.01%, 0.05%, and 0.1%. The reported enhancement of CHF was 103.37% for 0.1 vol% of r-GO-H₂O nanofluids. Shaafi et al. [24] numerically studied the boiling heat transfer performance using various techniques such as surface vibration, surface roughness, and coatings in a cylindrical chamber with three different water-based nanofluids (SiO₂, Al₂O₃, and ZrO₂) at volume percentages of 0.001%, 0.01%, and 0.1%. It was reported that the higher heat flux value (300 kW/m²) was found for 0.1 vol% Al₂O₃/water nanofluids. Improving boiling heat transfer efficiency is critical in various thermal systems, especially in heat pipes, electronics cooling, and power plants. Pool boiling provides greater heat transfer rates, while conventional fluids are limited in their thermal performance. The use of f-GnP with a controlled size and stable nanofluid, with its improved thermal conductivity and stability, could provide a favourable solution to overcome these issues and improve boiling heat transfer performance. This enhanced performance facilitates the development of an effective thermal management system for advanced engineering applications.

Extensive reports on the stability and pool boiling heat transfer behaviour of H₂O-based nanofluids are seen in the literature. H₂O-based metal/metal oxide nanofluids demonstrated limited enhancement and reduced stability due to their lower thermal conductivity and higher density compared to carbon-based nanomaterials, which have superior heat conductivity and lower density. Hence, using carbon-based nanomaterials, particularly those with a two-dimensional shape, and depositing nanomaterials on heating surfaces with nanofluids could enhance the CHF during pool boiling heat transfer. This approach, involving nanofluids with carbon-based nanomaterials, can be applied to small-scale applications like heat pipes in various systems. The literature reports the mechanisms responsible for the enhancement of CHF, which trigger an increase in the heat transfer rate, the formation of a porous layer on the surface, and a decrease in surface roughness, respectively. Hence, a good understanding of pool boiling with nanomaterials is essential in various research studies and applications, and further studies are needed to predict the exact boiling characteristics. This study experimentally investigates the pool boiling characteristics of f-GnP/d-H₂O nanofluids on Ni-Cr wire to enhance heat transfer performance and better understand surface interaction effects. Hence, in the current study, the pool boiling heat transfer characteristics, such as CHF and BHTC, of f-GnP-dH₂O nanofluids, heated through a Ni-Cr test wire, were studied experimentally. The f-GnP was selected in view of its excellent thermal characteristics, 2D nanomaterial shape, and lower density. Initially, the f-GnP-dH₂O nanofluids were prepared with variations of 0.01%, 0.025%, 0.05%, 0.075%, and 0.1%, and their stability was characterized by measuring the zeta potential distribution and pH. Finally, the CHF and BHTC were found through variations made to the electrical supply to the Ni-Cr test wire for the fixed vol% of f-GnP, including pure base fluid.

2. Materials and Methods

The choice of base fluid, selection of nanomaterial, preparation and characterization of the nanofluids, and the details of the experiment facility and experimental procedure, are discussed in this section.

2.1. Selection of Base Fluid and Nanomaterial

Distilled water (dH₂O) and functionalized graphene nanoplatelets (f-GnPs) were chosen as the base fluid and nanomaterial in this study. dH₂O was selected due to its substantial phase change characteristics, such as a higher boiling point and higher rate of condensation for boiling heat transfer applications. It is also a cost-effective liquid with higher thermal conductivity and stability than other traditional heat transfer fluids. GnP is a 2D-structured thin sheet of alloyed carbon that contains a honeycomb lattice structure, which was selected due to its greater heat conductivity, greater aspect ratio, and lower density compared to other types of nanomaterials. Furthermore, the dispersion of GnP into base fluids could improve its wettability, as reported in the literature [25,26,27]. The GnP was procured from Cheap Tubes, USA, with a thickness of 5–10 nm and a diameter of 15 µm. The visualization of scanning electron microscopy (SEM) confirmed the structure of the GnP, as shown in Figure 1.

2.2. Functionalization and Characterization of GnP

The purchased GnP was functionalized by acid treatment with concentrated nitric acid. The GnP was added to concentrated nitric acid and stirred well using a stirrer for 2 h at 400 rpm. The mixture was kept in an oil bath at 100 °C while mixing. Furthermore, the GnP/nitric acid mixture was subjected to reflux using a condenser. Then, the sample was thinned with the dH₂O. The GnP was filtered using the filter paper and cleaned with water until a pH value of 7 was obtained. Finally, the obtained GnP was dried in a furnace at a temperature of 150 °C for 6 h. The f-GnP was characterized by SEM, EDX, and XPS.

Various characterisations, such as SEM, EDX, and XPS, were carried out to find the surface functional groups in f-GnP. The results are illustrated in Figure 2a–c. The f-GnP flakes were separated properly, and the stack layers were found. The roughness in the edges increased, resulting in crumpled edges, as observed from the SEM visualization in Figure 2a. Furthermore, the f-GnPs were also confirmed using EDX (energy-dispersive X-ray spectroscopy), and it was found that the f-GnP consists of 92.96 wt% carbon (C) and 7.04 wt% oxygen (O), as illustrated in Figure 2b. The presence of reduced carbon (C) and improved oxygen (O) confirms the effective functionalization of GnP in the present study including the data presented in Figure 2c. Finally, the f-GnP was further characterized by XPS analysis to ensure the particle size. The thickness of GNP was found to be less than 4 nm, as provided by the manufacturer. No change in the thickness of GnP was observed after functionalization in the XPS study.

In pool boiling heat transfer systems, the incorporation of functionalized GnPs could improve interaction with the boiling surface by enhancing changes in surface morphology, which result in improved wettability. This extension of the nucleate boiling regime could postpone the transition to film boiling. Hence, the reduced vapour film accumulation and consistent fluid–surface interactions can result in enhanced critical heat flux (CHF) and a reduced boiling heat transfer coefficient (BHTC).

2.3. Preparation and Stability of Nanofluids

Initially, the purchased GnP was chemically functionalised and dispersed in the dH₂O as per the method described in our earlier examinations [28,29]. The chemically functionalised GnP was directly dispersed in the base fluid (dH₂O) using a probe sonicator (Labman Scientific Instruments, Chennai, India-PRO650) for an hour with variations made in its volume percentage, viz., 0.01%, 0.025%, 0.05%, 0.075%, 0.1%, and 0.125%, at an atmospheric pressure (1 atm) and atmospheric temperature in the range of 27–32 °C. The prepared nanofluid at higher concentration was further characterized by measuring pH (pH metre, LMPH15) and zeta potential immediately following the preparation (day 1) and after ten days (day 10) in order to find its stability at an ambient temperature, as shown in Figure 3a,b. The results show that the zeta potential values range from −31.6 mV to −30.6 mV, while the pH values range from 7.076 to 7.021 on day 1 and day 10, respectively. Hence, the prepared nanofluids were stable as evidenced by the zeta potential and pH results.

2.4. Measurements of Thermo-Physical Characteristics

The thermo-physical characteristics, namely thermal conductivity (k), viscosity (µ), specific heat capacity (Cp), and density (ρ), were measured using a KD2 Pro thermal analyzer (Decagon devices, Pullman, WA, USA), cone and plate viscometer (Lamy, Champagne au Mont d’Or, France), differential scanning calorimetry (DSC) (model: 214 Polyma, brand: NETZSCH, Selb, in Germany), and electronic weighing balance (Analytical balance pioneer, bishops scientific Pvt Ltd., Mumbai, India, accuracy: 1 mg) with variations made in temperatures ranging from 60 °C to 100 °C in steps of 10 °C. The methods used for measuring thermo-physical characteristics have been reported in our earlier studies [30].

2.5. Fabricated Test Facility for Exploratory Analysis

The schematic view of the fabricated test facility for exploratory analysis is shown in Figure 4. The fabricated test facility comprises a borosilicate glass container (2 litres capacity), pre-heater ((Genuine products, hot plate, 1500 watts, 220 volts, Bengaluru, India), copper electrodes, test wire (Ni-Cr, d = 0.2 mm, l = 150 mm), regulated power supply (15 amps), condenser (reflux-type, 2 litres capacity), constant temperature bath (1 TR), pump (capacity-10 m³/hr), temperature sensors (K-type thermocouple, ±0.1 °C accuracy), pressure gauge (range-0–1 kg/cm²), data acquisition system (Agilent-34970A, Keysight, Colorado Springs, CO, USA), and 3Nos. of DAQM901A-20 Channel Multiplexer (2/4-wire) Module. The borosilicate glass container was used to store the pure dH₂O and nanofluids to perform the pool boiling heat transfer study. The pre-heater was fixed under a glass container to heat the dH₂O to 100 °C. The copper electrodes were used to supply electrical energy to the test wire through a regulated power supply, as they were fixed in place. The condenser was fixed on the top side of the glass container to cool the dH₂O vapour, which became a liquid. The condensed liquid (dH₂O) was returned to the glass container again to prevent the loss of water. A constant-temperature water bath was used to circulate dH₂O at 25 °C through the pump to the condenser, cooling the dH₂O vapour at the top of the container. Two temperature sensors were installed in the container to measure the water temperature at various positions, and the data were logged in a data acquisition system. The pressure gauge was used to measure the pressure of water vapour to monitor and keep it constant. The photographic view of the fabricated test facility for exploratory is shown in Figure 5.

2.6. Process of Exploratory Analysis

Initially, the required quantity of dH₂O was placed in the glass container by submerging the Ni-Cr test wire in it. The dH₂O was heated by the pre-heater to reach its saturation point (i.e.,100 °C). The submerged test wire in a glass container was connected to copper electrodes with proper tension, and the electrical energy was supplied through the regulated power supply as shown in Figure 5. The electrical supply to the Ni-Cr test wire through the copper electrodes was supplied once the water reached its saturation point. The electrical energy was gradually increased until reaching the critical point of the Ni-Cr test wire, at which point it became reddish-yellow hot. The voltage and current were continuously logged in a regulated power supply as the electrical supply was increased at each step until reaching the Ni-Cr test wire critical point. The required water temperature in various locations was logged in a data acquisition system for reference and observation. The heat transfer rate (Q), CHF, and BHTC of the water were determined using the measured parameters, such as voltage (V) and current (I), from the experiments. The same experiments were repeated with the nanofluids for the considered vol% of f-GnP.

3. Data Reduction

The CHF of the Ni-Cr test wire in water was predicted using Equation (1) as given below.

$$q_{CHF}^{″}={\displaystyle \frac{V\times I}{A}}$$

(1)

where

I—current (Amps); V—voltage (Volts) across the wire.

A—surface area of the test heater wire = $\mathsf{\pi }\mathrm{d}\mathrm{L}$ (m²). d—diameter (mm); L—length (mm).

Initially, the typical system resistance (R_t) of Cu wire through the Cu rod was found to predict the resistance of the test wire. Electrical energy was supplied to the Cu rod by connecting the Cu wire through the regulated power supply, and the corresponding voltage and current were noted to find the typical system resistance. The study was repeated by replacing Cu wire with Ni-Cr wire to find the actual system’s resistance (R_a). Finally, the wire resistance (R_w) was predicted using Equation (2).

$$R_w=R_t-R_a$$

(2)

where

R_t—typical system resistance $R_t={\left[{\displaystyle \frac{V}{I}}\right]}_{Cu}$;

R_a—actual system resistance $R_a={\left[{\displaystyle \frac{V}{I}}\right]}_{Ni-Cr}$.

Initially, the base resistance (R_b) was determined using the temperature–resistance relationship to find the temperature of the test wire (T_w) following the procedure reported by Hyndman [31], as shown in Equations (2) and (3).

$$T_w=\left[\left({\displaystyle \frac{R_w}{R_b}}\right)-1\right]\left[{\displaystyle \frac{1}{\propto }}\right]+T_b$$

(3)

where T_w—wire temperature; T_b—baseline temperature (saturation temperature 100 °C); R_w—wire resistance; R_b—baseline resistance of the wire; and ∝—coefficient of resistance.

4. Uncertainty

Uncertainties were predicted with the measured parameters using the approach proposed by Zafar et al. [32] and Sivashankar et al. [33]

Uncertainties in heat flux were predicted using the measured parameters in Equation (4).

$${\displaystyle \frac{Uq}{q}}=\pm \sqrt{{\left({\displaystyle \frac{UI}{q}}{\displaystyle \frac{\partial q}{\partial I}}\right)}^2+{\left({\displaystyle \frac{UV}{q}}{\displaystyle \frac{\partial q}{\partial V}}\right)}^2+{\left({\displaystyle \frac{UD}{q}}{\displaystyle \frac{\partial q}{\partial D}}\right)}^2+{\left({\displaystyle \frac{UL}{q}}{\displaystyle \frac{\partial q}{\partial L}}\right)}^2}$$

(4)

The voltage and current data for each experiment case were logged in a data acquisition system with ±1% accuracy. The uncertainty in heat flux was found to be ~$\pm 2.57\mathrm{\%}$. Similarly, uncertainties in the wire resistance (R_w), baseline resistance (R_b), and wire temperature (T_w) were predicted using Equations (5)–(7):

$${\displaystyle \frac{U{R}_w}{R_w}}=\pm \sqrt{{\left({\displaystyle \frac{{UV}_w}{R_w}}{\displaystyle \frac{{\partial R}_w}{{\partial V}_w}}\right)}^2+{\left({\displaystyle \frac{{UI}_w}{R_w}}{\displaystyle \frac{{\partial R}_w}{{\partial I}_w}}\right)}^2}=\pm 0.312\%$$

(5)

$${\displaystyle \frac{{UR}_b}{R_b}}=\sqrt{{\left({\displaystyle \frac{{UV}_b}{R_b}}{\displaystyle \frac{{\partial R}_b}{{\partial V}_b}}\right)}^2+{\left({\displaystyle \frac{{UI}_b}{R_b}}{\displaystyle \frac{{\partial R}_b}{{\partial I}_b}}\right)}^2}=\pm 0.259\%$$

(6)

$${\displaystyle \frac{{UT}_w}{T_w}}=\pm \sqrt{{\left({\displaystyle \frac{{UR}_w}{T_w}}{\displaystyle \frac{{\partial T}_w}{{\partial R}_w}}\right)}^2+{\left({\displaystyle \frac{{UR}_b}{T_w}}{\displaystyle \frac{{\partial T}_w}{{\partial R}_b}}\right)}^2+{\left({\displaystyle \frac{{UT}_b}{T_w}}{\displaystyle \frac{{\partial T}_w}{{\partial T}_b}}\right)}^2}=\pm 0.99\%$$

(7)

The uncertainties in wire resistance (R_w), baseline resistance (R_b), and wire temperature (T_w) were observed to be $\pm$0.312%, $\pm$0.259%, and $\pm$0.99%, respectively.

Uncertainties in BHTC were predicted with the measured parameters given in Equation (8).

$${\displaystyle \frac{Uh}{h}}=\pm \sqrt{{\left({\displaystyle \frac{Uq}{q}}\right)}^2+{\left({\displaystyle \frac{{UT}_w}{T_w-T_b}}\right)}^2+{\left({\displaystyle \frac{{UT}_b}{T_w-T_b}}\right)}^2}=\pm 1.76\%$$

(8)

The uncertainty in BHTC was found to be ~$\pm 1.76\mathrm{\%}$. Hence, the uncertainties in the present study indicate a higher level of confidence, suggesting that the coverage factor is k = 4.3.

5. Results and Discussion

The pool boiling heat transfer characteristics of dH₂O and f-GnP-dH₂O nanofluids when heating through a Ni-Cr test wire surface were experimentally investigated in the current study for various vol% of f-GnP. The electrical supply provided to the Ni-Cr test wire surface was increased until it became reddish-yellow hot. The CHF and BHTC were predicted with the experimental inputs. The variations in the results of the CHF and BHTC with respect to various vol% of f-GnP are discussed in this section.

5.1. Heat Flux vs. Excess Temperature for Various Volume Concentrations

Initially, the heat flux of dH₂O was predicted with the experimentally supplied voltage and current to the test wire through the Cu electrode of the test facility. Then, the experimental heat flux of dH₂O with the Ni-Cr test wire was compared with the literature data for the same base fluid reported by Milanova and Kumar [17], as illustrated in Figure 6. The experimental results remained in good agreement with the literature results, showing a 10% deviation.

The variations in heat flux with excess temperature (ΔT) for different vol% of f-GnP are shown in Figure 7. Initially, the heat flux increases with ΔT, but it decreases within the nucleate boiling region of vol% f-GnP. The average increments in heat flux were found to be ~60% when the ΔT increased from 1 °C to 200 °C, while the CHF was found to be ~90% when the ΔT increased from 200 °C to 1400 °C at the melting point of the Ni-Cr test wire. It was observed that the heat flux increases with vol% f-GnP in a film boiling region due to an increment of delay in the nucleate boiling region. The maximum enhancements in CHF were observed to be ~15% before the transition boiling and ~24% during the burnout point, respectively, at 0.1 vol% compared to pure dH₂O. Furthermore, the heat flux was increased gradually while varying the excess temperature from 5 °C to 200 °C up to the nucleate boiling region. Then, the heat flux with dH₂O rapidly increased when the excess temperature increased from 200 °C to above 1000 °C due to the transition boiling region and the rapid formation of a film on the Ni-Cr test wire. The use of the Ni-Cr test wire could be the reason for the rapid increment in heat flux due to the absence of the power-controlled mode during the transition boiling region [5,34]. f-GnP was added to dH₂O to delay the transition boiling region, which resulted in a reduction in the formation of film on the Ni-Cr test wire. Hence, the addition of f-GnP to dH₂O significantly reduces the formation of film on the test surface, which increases the heat flux. Furthermore, the deposition of f-GnP on the surface of the Ni-Cr test wire could be another reason for the enhancement.

The variation in CHF with vol% f-GnP and its enhancement at the point before breaking of the Ni-Cr test wire are depicted in Figure 8a,b. The results show a significant increase in CHF with increases in the vol% of f-GnP compared to pure dH₂O. The CHF increased from 1164 kW/m² to 1195 kW/m² when the vol% increased from 0 to 0.01%, while it increased from 1164 kW/m² to 1234 kW/m² when the vol% increased from 0% to 0.025%, as indicated in Figure 8a. The minimum increment in CHF was ~3% with the addition of 0.01% in dH₂O, while the average enhancement in CHF was ~2–5% for every increase in 0.025% vol%. A significant increase of 13.39% in CHF was observed with the addition of f-GnP to dH₂O up to 0.075 vol%, while a smaller enhancement of ~2.5% was observed for every addition of 0.025 vol% in dH₂O. Furthermore, the CHF was found to be 0.125 vol%, with an observed enhancement of up to 16%. It was also found that the CHF is limited beyond 0.1 vol%. Hence, this study was limited to 0.1 vol% of f-GnP to prevent excessive deposition on the surface of the Ni-Cr test wire, which could reduce performance [23].

The improved oxygen content in the f-GnP is observed in the SEM and EDX results, along with an increase in roughness, which could enhance surface wettability. This improvement helps in delaying the formation of vapour blankets, thereby increasing the CHF. GnP has a thin sheet structure and higher thermal conductivity, which improves microlayer evaporation between vapour bubbles and the heater surface by efficiently removing heat. This could be the possible reason for the enhancement in CHF. The interaction between liquids and vapour, which could stabilize the boiling process, is also a possible factor for an increase in the CHF. Moreover, the CHF was found to be 1164 kW/m² for d-H₂O (0 vol%), while the improved CHF was found to be 1350 kW/m² for 0.1 vol% f-GnP/d-H₂O nanofluids in the present results. The values of CHF reported in the literature [14,19,34,35] are 530 kW/m², 550 kW/m², and 1070 kW/m² for 0 vol%, while the improved CHF values were reported to be 600kW/m² (0.5–4 vol%, Al₂O₃-H₂O), 850 kW/m² (0.05 vol% Al₂O₃ -EG/DW), 1160 kW/m² (0.25 wt%, Cu-H₂O), and 740 kW/m² (0.1–0.8 vol%, GNPs-SiO₂), respectively, with different nanofluids. The summary of the comparison is presented in

Table 1. A summary of the comparison.

Author	Nanoparticles	Vol%	Heat Flux (kW/m2)
			Minimum	Maximum
Present study	f-GnP	0.01–0.1%	1164	1350
Bang & Chang [14]	Alumina	0.5–4%	530	600
Ajeeb & Murshed [19]	Al2O3	0.05%	550	850
Kathiravan et al. [35]	Cu	0.25 wt %	1070	1160
Huang et al. [36]	GNPs-SiO2	0.1–0.8%	700	740

.

5.2. Boiling Heat Transfer Coefficient

The variations in BHTC as a function of heat flux for various vol% of f-GnP in the nucleate boiling region are illustrated in Figure 9. A decrease in the BHTC was seen with an increase in heat flux and vol% of f-GnP. The average decrement in BHTC was found to be in the range of ~40% to ~90% for every increase in 200 kW/m² heat flux. Initially, the BHTC decreased from ~10% to ~60% as the vol% of f-GnP increased from 0% to 0.01%, and the decrement increased by up to 25% when the vol% increased from 0% to 0.025%. The maximum increments seen in BHTC were ~12% at a 1350 kW/m² heat flux and 0.1 vol% well before the breaking point of the test wire. The decrement in BHTC was found to be ~25% (at ~1.8 kW/m²K) when the vol% of f-GnP increased in successive steps of 0.025%, which is significant. The large cavities on the Ni-Cr test wire formed during the pool boiling of pure H₂O at a lower heat flux, while small cavities formed at higher heat flux ranges. The cavities were filled with nanomaterials, and the use of nanofluids resulted in a reduction in BHTC. The decrease in the BHTC with the use of f-GnP nanofluids is attributed to changes in surface cavities, as explained by the cavity theory. The use of f-GnP nanofluids in the boiling process on the heating surface can obstruct the active nucleation sites, thereby reducing the bubble formation as per the cavity theory. The surface energy (wettability) was enhanced due to the functionalization of GnP, which delays the surface’s drying process and makes it easier to rewet. These factors result in a reduction in the BHTC with respect to the vol% of f-GnP.

5.3. SEM Visualization of Nichrome Wire After the Boiling of dH₂O and f-GnP-dH₂O Nanofluids

The SEM visualization of Ni-Cr wire surfaces was performed following the boiling of pure base fluid and f-GnP-dH₂O nanofluids with various vol% of f-GnP, as illustrated in Figure 10a–d. Higher roughness was found on the surface of the Ni-Cr test wire with the heating of pure base fluid, while lower roughness was seen during the heating of nanofluids. The roughness of the Ni-Cr test wire continuously decreased while the vol% f-GnP increased, as evidenced from the SEM visualization shown in Figure 10b–d. Thus, the CHF was higher with the use of nanofluids at higher vol% due to a decrease in surface roughness of the Ni-Cr test wire and higher thermal conductivity of the nanofluid. Moreover, porous layers might form on the surface of the Ni-Cr test wire due to the dispersion of f-GnP in dH₂O, resulting in an improvement in the duration of the nucleate boiling region [20]. The large quantities of smaller bubbles formed during the extended nucleate boiling region increased the CHF when using nanofluids at higher vol% of f-GnP.

The mechanisms responsible for the present results are discussed. The addition of GnP delays the transition boiling zone, reduces film formation due to microscopic surface modifications, and promotes nanoparticle deposition during the boiling process. Surface roughness and porous nanostructures with nanofluid were observed in the SEM results, acting as effective nucleation sites and promoting liquid rewetting. The formation of a liquid film region across the heated surface maintains constant liquid contact and prevents early vapour film formation, which also delays the onset of transition boiling. Based on the mechanisms observed from the SEM analysis, the changes in surface morphology can effectively contribute to the enhancement of the CHF, as shown in Figure 11.

6. Summary and Conclusions

The pool boiling heat transfer characteristics of dH₂O and f-GnP–dH₂O nanofluids, measured by heating through a Ni-Cr test wire in the CHF, were studied and are reported in the current study. The conclusions obtained from the current experimental study are listed below.

• The f-GnP–dH₂O nanofluids with various vol% were characterized by measuring zeta potential distribution and pH to ensure stability on day 1 and day 10 following preparation.
• The results show zeta potential values ranging from −31.6 mV to −30.6 mV and pH values ranging from 7.076 to 7.021 between day 1 and day 10, respectively, which confirm the good stability of f-GnP–dH₂O nanofluids.
• The average increments in heat flux were found to be ~60% when the ΔT increased from 1 °C to 200 °C, while it was found to be ~90% when the ΔT increased from 200 °C to 1400 °C at a critical point. The maximum enhancements in critical heat flux were observed to be ~15% before the transition boiling region and ~24% at the burnout point of the Ni-Cr test wire in the film boiling region at 0.1 vol%, respectively, compared to dH₂O.
• A significant 13.39% increment of up to 0.075 vol% in CHF was observed with the addition of f-GnP to dH₂O, while smaller enhancements of ~ 2.5% were observed after the successive addition of 0.025% in dH₂O.
• The average decrement in the BHTC was found to be in the range of ~40% to ~90% for every increase in 200 kW/m² heat flux. Initially, the BHTC decreased from ~10% to ~60% when the vol% of f-GnP increased from 0% to 0.01%, while the decrement increased by up to 25% when the vol% increased from 0 to 0.025%.
• The maximum increment in BHTC was found to be ~12% at a 0.1 vol% of f-GnP–dH₂O nanofluid.
• The SEM results show higher roughness on the surface of the Ni-Cr test wire with the heating of pure base fluid, while lower roughness was found with the heating of nanofluids.

7. Scope for Future Works

The porous layers formed on the surface of the Ni-Cr test wire due to the dispersion of f-GnP in dH₂O could improve surface wettability, thereby enhancing the duration of the nucleate boiling region. The present study was limited with 0.1 vol% of GnP to avoid its excessive deposition on the surface of the Ni-Cr test wire, which could reduce performance. Hence, the present study could be a foundation to enhance the two-phase flow characteristics with an increase in the CHF for various heat pipe applications. Heat pipe technology has an excellent scope with the use of nanofluids and micro-fabrication in its design. Heat pipes with higher heat conduction and heat flux are commonly used to improve performance in applications like energy systems, electronics cooling, and various emerging trends. The use of dH₂O-based chemically functionalized carbon nanofluids could enhance heat conduction in heat pipes, thereby increasing their critical heat flux, as demonstrated in the present work. The use of chemically functionalised carbon-based two- and one-dimensional structured dH₂O-based hybrid nanofluids, using micro-fabrication in heat pipes without surfactant, could be the scope of a future study.