Flow boiling in integrated microchannel systems is a cooling technology that has received significant attention in recent years as an effective option for high heat flux microelectronic devices as it provides high heat transfer and small variations in surface temperature. However, there are still a number of issues to be addressed before this technology is used for commercial applications. Amongst the issues that require further investigation are the two-phase heat transfer enhancement mechanisms, the effect of channel geometry on heat transfer characteristics, two-phase flow instabilities, critical heat flux and interfacial liquid-vapour heat transfer in the vicinity of the wall. This work is an experimental study on two-phase flow boiling in multi- and single-rectangular microchannels. Experimental research was performed on the effect of the channel aspect ratio and hydraulic diameter, particularly for parallel multi-microchannel systems in order to provide design guidelines. Flow boiling experiments were performed using deionised water in silicon microchannel heat sinks with width-to-depth aspect ratios (a) from 0.33 to 3 and hydraulic diameters from 50 μm to 150 μm. The effect of aspect ratio on two-phase flow boiling local heat transfer coefficient and two-phase pressure drop was investigated as well as the two-phase heat transfer coefficients trends with mass flux for the constant heat fluxes of 151 kW m-2, 183 kW m- 2, 271 kW m-2 and 363 kW m-2. Wall temperature measurements were obtained from five integrated thin nickel film temperature sensors. An integrated thin aluminium heater enabled uniform heating with a small thermal resistance between the heater and the channels. The microfabricated temperature sensors were used with simultaneous high-speed imaging and pressure measurements in order to obtain a better insight related to temperature and pressure fluctuations caused by two-phase flow instabilities under uniform heating in parallel microchannels. The results demonstrated that the aspect ratio of the microchannels affects flow boiling heat transfer coefficients. However, there is not clear trend of the aspect ratio on the heat transfer coefficient. Pressure drop was found to increase with increasing aspect ratio. Wide microchannels but not very shallow, with a = 1.5 and Dh = 120 μm, have shown good heat transfer performance, by producing modest two-phase pressure drop of maximum 200 mbar for the highest heat flux and heat transfer coefficients of 200 kW m-2 during two-phase flow boiling conditions. For the high aspect ratio, values of 2 and 3 two-phase flow boiling heat transfer coefficients were measured to be lower compared to aspect ratio of 1.5. Microchannels with aspect ratios higher than 1.5 produced severe wall temperature fluctuations for high heat fluxes that periodically reached extreme wall temperature values in excess of 250 ˚C. The consequences of these severe wall temperature and pressure fluctuations at high aspect ratios of 2 and 3 resulted in non-uniform flow distribution and temporal dryout. Abrupt increase in two-phase pressure drop occurred for a > 1.5. The effect of the inlet subcooling was found to be significant on both heat transfer coefficient and pressure drop. Furthermore, the effects of bubble growth on flow instabilities and heat transfer coefficients have been investigated. Although the thin film nickel sensors provide the advantage of much faster response time and smaller thermal resistance compared to classic thermocouples, they do not allow for full two-dimensional wall temperature mapping of the heated surface. An advanced experimental method was devised in order to produce accurate two-dimensional heat transfer coefficient data as a function of time. Infrared (IR) thermography was synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a polydimethylsiloxane (PDMS) microchannel. The microchannel was characterised by a high aspect ratio (a = 22) and a hydraulic diameter of 192 μm. The PDMS microchannel was bonded on a transparent indium tin oxide (ITO) thin film coated glass. The transparent thin film ITO heater allowed the recording of high quality synchronised high - speed images of the liquid-vapour distribution. This work presents a better insight into the two-phase heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during the cycles of liquid-vapour alternations for different mass flux and heat flux conditions. High spatial and temporal resolution wall temperature measurements and pressure data were obtained for a range of mass fluxes from 7.37 to 298 kg m-2 s-1 and heat fluxes from 13.64 to 179.2 kW m-2 using FC-72 as a dielectric liquid. 3D plots of spatially averaged two-phase heat transfer coefficients at the inlet, middle and outlet of the microchannel are presented with time. The optical images were correlated, with simultaneous thermal images. The results demonstrate that bubble growth in microchannels differs from macroscale channels and the confinement effects influence the local two-phase heat transfer coefficient distribution. Bubble nucleation and axial growth as well as wetting and rewetting in the channel were found to significantly affect the local heat transfer physical mechanisms. Bubble level heat transfer coefficient measurements are important as previous researchers have experimentally investigated local temperature and high speed visualisation in bubbles during pool boiling conditions and not flow boiling. The effect of the confined bubble axial growth to the two-phase heat transfer coefficient distribution at the channel entrance was investigated at low mass fluxes and low heat fluxes. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with liquid-vapour dynamics and liquid film thinning from the contrast of the optical images, which caused suspected dryout. The 3D plots of heat transfer coefficients with time provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow patterns. The correlation between the synchronised high-resolution thermal and optical images assisted in a better understanding of the heat transfer mechanisms and critical heat flux during two-phase flow boiling in microchannels.

Two-phase flows with or without phase change are present in terrestrial and space applications like thermal control of satellites, propellant supply for launchers, and waste water treatment for space exploration missions. Flow boiling experiment with HFE7000 were conducted in a heated tube in vertical upward flow on ground and in microgravity conditions to collect data on flow patterns, pressure drops, heat transfers, void fraction. Void fraction measurements allowed to measure mean gas velocity and the liquid film thickness in annular flow. In microgravity condition, the liquid film thickness and the interfacial shear stress are significantly lower than in normal gravity. A detail analysis of the film structure was performed by image processing. The impact of gravity and liquid and vapour superficial velocities on the disturbance waves velocities and frequencies was investigated. Two different measurement techniques were used and compared to determine the heat transfer coefficient. For quality values greater than 0.2, HTC is not sensitive to gravity and is in good agreement with classical correlations of the literature. For qualities smaller than 0.1, in the subcooled nucleate boiling regime HTC is significantly smaller in microgravityconditions.

Boiling in microchannels is a very efficient mode of heat transfer with high heat and mass transfer coefficients achieved. Less pumping power is required for two-phase flows than for single-phase liquid flows to achieve a given heat removal. Applications include electronics cooling such as cooling microchips in laptop computers, and process intensification with compact evaporators and heat exchangers. Evaporation of the liquid meniscus is the main contributor to the high heat fluxes achieved due to phase change at thin liquid films in a microchannel. The microscale hydrodynamic motion at the meniscus and the flow boiling heat transfer mechanisms in microchannels are not fully understood and are very different from those in macroscale flows. Flow instability phenomena are noted as the bubble diameter approaches the channel diameter. These instabilities need to be well understood and predicted due to their adverse effects on the heat transfer. A fundamental approach to the study of two-phase flow boiling in microchannels has been carried out. Simultaneous visualisation and hydrodynamic measurements were carried out investigating flow boiling instabilities in microchannels using two different working fluids (n-Pentane and FC-72). Rectangular, borosilicate microchannels of hydraulic diameter range 700-800 μm were used. The novel heating method, via electrical resistance through a transparent, metallic deposit on the microchannel walls, has enabled simultaneous heating and visualisation to be achieved. Images and video sequences have been recorded with both a high-speed camera and an IR camera. Bubble dynamics, bubble confinement and elongated bubble growth have been shown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodic instabilities have been observed during flow boiling in the microchannel. Analysis of the IR images in conjunction with pressure drop readings, have allowed the correlation of the microchannel pressure drop to the wall temperature profile, during flow instabilities. Bubble size is an important parameter when understanding boiling characteristics and the dynamic bubble phenomena. In this thesis it has been demonstrated that the flow passage geometry and microchannel confinement effects have a significant impact on boiling, bubble generation and bubble growth during flow boiling in microchannels.

<div>The continued miniaturization and demand for improved performance of electronic devices has resulted in the need for transformative thermal management strategies. Flow boiling is an attractive approach for the thermal management of devices generating high heat fluxes. However, designing heat sinks for two-phase operation and predicting their performance is difficult because of, in part, commonly encountered flow boiling instabilities and a lack of experimentally validated physics-based phase change models. This work aims to advance the state of the art by furthering our understanding of flow boiling instabilities and their implications on the operating characteristics of electronic devices. This is of particular interest under transient and non-uniform heating conditions because of recent advancements in embedded cooling techniques, which exacerbate spatial non-uniformities, and the demand for cooling solutions for next-generation electronic devices. Additionally, this work aims to provide a high-fidelity experimental characterization technique for slug flow boiling to enable the validation of physics-based phase change models.</div><div><br></div><div>To provide a foundation for which the effects of transient and non-uniform heating can be studied, flow boiling instabilities are first studied experimentally in a single, 500 μm-diameter borosilicate glass microchannel. A thin layer of optically transparent and electrically conductive indium tin oxide coated on the outside surface of the microchannel provides a spatially uniform and temporally constant heat flux via Joule heating. The working fluid is degassed, dielectric HFE-7100. Simultaneous high-frequency measurement of reservoir, inlet, and outlet pressures, pressure drop, mass flux, inlet and outlet fluid temperatures, and wall temperature is synchronized to high-speed flow visualizations enabling transient characterization of the thermal-fluidic behavior.</div><div><br></div><div>The effect of flow inertia and inlet liquid subcooling on the rapid-bubble-growth instability at the onset of boiling is assessed first. The mechanisms underlying the rapid-bubble-growth instability, namely, a large liquid superheat and a large pressure spike, are quantified. This instability is shown to cause flow reversal and can result in large temperature spikes due to starving the heated channel of liquid, which is especially severe at low flow inertia.</div><div><br></div><div>Next, the effect of flow inertia, inlet liquid subcooling, and heat flux on the hydrodynamic and thermal oscillations and time-averaged performance is assessed. Two predominant dynamic instabilities are observed: a time-periodic series of rapid-bubble-growth instabilities and the pressure drop instability. The heat flux, ratio of flow inertia to upstream compressibility, and degree of inlet liquid subcooling significantly affect the thermal-fluidic characteristics. High inlet liquid subcoolings and low heat fluxes result in time-periodic transitions between single-phase flow and flow boiling that cause large-amplitude wall temperature oscillations and a time-periodic series of rapid-bubble-growth instabilities. Low inlet liquid subcoolings result in small-amplitude thermal-fluidic oscillations and the pressure drop instability. Low flow inertia exacerbates the pressure drop instability and results in large-amplitude thermal-fluidic oscillations whereas high flow inertia reduces their severity.</div><div><br></div><div>Flow boiling experiments are then performed in a parallel channel test section consisting of two thermally isolated, heated microchannels to study the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power. The experimentally observed temperature excursion between the channels due to the Ledinegg instability is reported here for the first time.</div><div><br></div><div>Time-resolved characterization of flow boiling in a single microchannel is then performed during transient heating conditions. For transient heating tests, three different heat flux levels are selected that exhibit highly contrasting flow behavior during constant heating conditions: a low heat flux corresponding to single-phase flow (15 kW/m²), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m²), and a very high heat flux which would cause critical heat flux if operated at this heat flux continuously (150 kW/m²). Transient testing is first conducted using a single heat flux pulse between these heat flux levels and varying the pulse time. It is observed that any step up/down in the heat flux level that induces/ceases boiling, causes the temperature to temporarily over/under-shoot the eventual steady temperature. Following the single heat flux pulse experiments, a time-periodic series of heat flux pulses is applied. A square wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m²). Three different time-periodic flow boiling fluctuations are observed: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux.</div><div><br></div><div>To improve our ability to predict the performance of heat sinks for two-phase operation, high-fidelity characterization of key hydrodynamic and heat transfer parameters during microchannel slug flow boiling is performed using a novel experimental test facility that generates an archetypal flow regime, devoid of flow instabilities and flow regime transitions. High-speed flow visualization images are analyzed to quantify the uniformity of the vapor bubbles and liquid slugs generated, as well as the growth of vapor bubbles over a range of heat fluxes. A method is demonstrated for measuring liquid film thickness from the visualizations using a ray-tracing procedure to correct for optical distortions. Characterization of the slug flow boiling regime that is generated demonstrates the unique ability of the facility to precisely control and quantify hydrodynamic and heat transfer characteristics.</div><div><br></div><div>This work has advanced state-of-the-art technologies for the thermal management of high-heat-flux-dissipation devices by providing an improved understanding on the effects of transient and non-uniform heating on flow boiling and an experimental method for the validation of physics-based flow boiling modeling.</div>

Este trabalho trata do estudo teórico do ebulição convectiva de nanofluidos em canais de diâmetro reduzido (denominados de microcanais). Ele aborda, primeiramente, uma análise da literatura sobre a ebulição convectiva de fluidos convencionais em microcanais, na qual são discutidos critérios para a transição entre macro e microcanais e os padrões de escoamentos observados em canais de reduzido diâmetro. Métodos para a previsão das propriedades de transporte de nanofluidos foram levantados da literatura e estudos experimentais da convecção forçada, da ebulição nucleada e da ebulição convectiva de nanofluidos foram discutidos. Um método para a previsão do coeficiente de transferência de calor de nanofluidos em microcanais durante a ebulição convectiva foi proposto baseado em modelos convencionais da literatura ajustados para nanofluidos. O ajuste dos modelos convencionais foi realizado através de análise regressiva de dados experimentais para ebulição nucleada e convecção forçada de nanofluidos levantados da literatura, e da análise crítica de adimensionais que capturassem a influência das nanopartículas no processo de transferência de calor. De maneira geral o método proposto neste estudo apresenta concordância razoável com dados experimentais independentes, referente ao acréscimo do coeficiente de transferência de calor com o incremento da concentração volumétrica de nanopartículas. No entanto, a escassez de estudos experimentais sobre a ebulição convectiva de nanofluidos, especialmente em microcanais, impossibilitou uma análise mais aprofundada do método proposto. / The present work aims the theoretical study of convective boiling of nanofluids in small diameter channels (called microchannel). It discusses an analysis of the literature on convective boiling of conventional fluids in microchannels which presents criteria for the transition between conventional and microchannels and the flow patterns observed in small diameter channels. Methods for predicting the transport properties of nanofluids were compiled from the literature and experimental studies of forced convection, nucleate boiling and convective boiling of nanofluids were discussed. A method for predicting the heat transfer coefficient of nanofluids in microchannels during convective boiling was proposed based on conventional models from literature adjusted to nanofluids. The conventional models fitting was performed by regression analysis of experimental data for nucleate boiling and forced convection of nanofluids compiled from the literature and by critical analysis of dimensionless numbers which enable to capture the influence of nanoparticles on heat transfer process. In general the proposed method in this work presents reasonable agreement with independent experimental data regarding the increase in heat transfer coefficient with increasing nanoparticles volume fraction. However the scarcity of experimental studies on the convective boiling of nanofluids, especially in microchannels, precluded further analysis of the proposed method.

The objective of this investigation was to develop micro/nano-scale temperature sensors for measuring surface temperature transients in multi-phase flows and heat transfer. Surface temperature fluctuations were measured on substrates exposed to phase change processes. Prior reports in the literature indicate that these miniature scale surface temperature fluctuations can result in 60-90 percent of the total heat flux during phase change heat transfer. In this study, DTS (Diode Temperature Sensors) were fabricated with a doping depth of ~100 nm on n-type silicon to measure the surface temperature transients on a substrate exposed to droplet impingement cooling. DTS are expected to have better sensor characteristics compared to TFTs (Thin Film Thermocouples), due to their small size and faster response (which comes at the expense of the smaller operating temperature range). Additional advantages of DTS include the availability of robust commercial micro fabrication processes (with diode and transistor node sizes currently in the size range of ~ 30 nm), and that only 2N wire leads can be used to interrogate a set of N x N array of sensors (in contrast thermocouples require 2 N x N wire leads for N x N sensor array). The DTS array was fabricated using conventional semi-conductor processes. The temperature response of the TFT and DTS was also calibrated using NIST standards. Transient temperature response of the DTS was recorded using droplet impingement cooling experiments. The droplet impingement cooling experiments were performed for two different test fluids (acetone and ethanol). An infrared camera was used to verify the surface temperature of the substrate and compare these measurements with the temperature values recorded by individual DTS. PVD (Physical Vapor Deposition) was used for obtaining the catalyst coatings for subsequent CNT synthesis using CVD (Chemical Vapor Deposition) as well as for fabricating the thin film thermocouple (TFT) arrays using the "lift-off" process. Flow boiling experiments were conducted for three different substrates. Flow boiling experiments on bare silicon wafer surface were treated as the control experiment, and the results were compared with that of CNT (Carbon Nano-Tube) coated silicon wafer surfaces. Similar experiments were also performed on a pure copper surface. In addition, experiments were performed using compact condensers. Micro-scale patterns fabricated on the refrigerant side of the compact heat exchanger were observed to cause significant enhancement of the condensation heat transfer coefficient.

An experimental investigation was carried to examine the flow/ thermal field characteristics with/without phase change in the microchannels and compared with the traditional results. There are three parts in this study. The first part investigated the 2-D flow field measured by the micro particle image velocimetry (£gPIV) in a single PMMA microchannel fabricated by an ArF excimer laser. The slip boundary condition in the microchannel wall was also discussed. The second part studied the influence of surface condition (hydrophilic vs hydrophobic) on the flow/thermal field in a micro cooling device which included twenty parallel microchannels, which was fabricated by SU-8 microfabrication technique and replicated by the PDMS replica technique. The UV/ozone device was used to change the PDMS microchannels¡¦ surface condition from hydrophobic to hydrophilic and the £gPIV/£gLIF system was also used to measure the velocity and temperature distribution. The third part investigated the two-phase subcooled flow boiling phenomena (onset of nucleate boiling, boiling curve, flow patterns, bubble departure diameter and frequency) in the seventy-five parallel microchannels fabricated by SU-8 microfabrication technique, and aimed to raise the critical heat flux (CHF) and heat transfer coefficient to enhance the cooling efficiency. Three major methods were used in this study, as follows: (1) To add the cavity angle of £c = 60¢X, 90¢X, and 120¢X on the microchannel side walls. (2) To coat 2 £gm diamond film on the Cu heated surface. (3) To add 1 vol. % Multi-walled Carbon Nanotube (MCNT) into the working medium (deionized water). The goal of this paper is to develop a high heat flux cooling technique and apply the experimental results to solve the cooling problem resulting from the exceedingly high heat flux from the electronic component.

Este trabalho trata do estudo teórico do ebulição convectiva de nanofluidos em canais de diâmetro reduzido (denominados de microcanais). Ele aborda, primeiramente, uma análise da literatura sobre a ebulição convectiva de fluidos convencionais em microcanais, na qual são discutidos critérios para a transição entre macro e microcanais e os padrões de escoamentos observados em canais de reduzido diâmetro. Métodos para a previsão das propriedades de transporte de nanofluidos foram levantados da literatura e estudos experimentais da convecção forçada, da ebulição nucleada e da ebulição convectiva de nanofluidos foram discutidos. Um método para a previsão do coeficiente de transferência de calor de nanofluidos em microcanais durante a ebulição convectiva foi proposto baseado em modelos convencionais da literatura ajustados para nanofluidos. O ajuste dos modelos convencionais foi realizado através de análise regressiva de dados experimentais para ebulição nucleada e convecção forçada de nanofluidos levantados da literatura, e da análise crítica de adimensionais que capturassem a influência das nanopartículas no processo de transferência de calor. De maneira geral o método proposto neste estudo apresenta concordância razoável com dados experimentais independentes, referente ao acréscimo do coeficiente de transferência de calor com o incremento da concentração volumétrica de nanopartículas. No entanto, a escassez de estudos experimentais sobre a ebulição convectiva de nanofluidos, especialmente em microcanais, impossibilitou uma análise mais aprofundada do método proposto. / The present work aims the theoretical study of convective boiling of nanofluids in small diameter channels (called microchannel). It discusses an analysis of the literature on convective boiling of conventional fluids in microchannels which presents criteria for the transition between conventional and microchannels and the flow patterns observed in small diameter channels. Methods for predicting the transport properties of nanofluids were compiled from the literature and experimental studies of forced convection, nucleate boiling and convective boiling of nanofluids were discussed. A method for predicting the heat transfer coefficient of nanofluids in microchannels during convective boiling was proposed based on conventional models from literature adjusted to nanofluids. The conventional models fitting was performed by regression analysis of experimental data for nucleate boiling and forced convection of nanofluids compiled from the literature and by critical analysis of dimensionless numbers which enable to capture the influence of nanoparticles on heat transfer process. In general the proposed method in this work presents reasonable agreement with independent experimental data regarding the increase in heat transfer coefficient with increasing nanoparticles volume fraction. However the scarcity of experimental studies on the convective boiling of nanofluids, especially in microchannels, precluded further analysis of the proposed method.

Presented here are the results from the steady state flow boiling of R245FA in a laboratory scale horizontal stainless-steel test tube with an inner diameter of 8.5 mm and a length of 900 mm at a saturation temperature of 35 °C and 40 °C. Experiments were conducted at mass fluxes ranging between 200 and 300 kg/m²s at inlet vapour qualities from 0.2 to 0.7 under uniform, and non-uniform imposed heat flux cases that are expected to exist in horizontal parabolic trough solar collectors. Nine (9) different heat flux distributions were investigated. Local and average heat transfer coefficients (HTC’s) were determined based on wall temperature measurements taken along the length and around the circumference of the test section. Through the choice of the fluid being linked to the possible usage of DSG technology in organic Rankine cycles, the qualitative trends and observed performance variations can be used to predict the same for a working fluid such as water. It was found that the non-uniformity of the heat flux greatly alters the HTC’s of the fluid undergoing boiling but has no effect on the pressure drop characteristics of the fluid undergoing boiling. Heating only on the sides of the tube yielded HTC’s that were 46 % lower than achieved under uniform heating. Heating only from the top proved to be more effective in heat transmission to the fluid than heating only from the bottom (as is the case on PTC solar fields), by only a slight margin, and both these cases yielded HTC’s that were 30 % lower than the uniform heating case. Applying a bell curve heat flux distribution over the tube walls yielded overall HTC’s that differed from the uniform case by a maximum of 5 %, even as the peak heat flux position changes around the circumference of the tube. A further study may be done to quantify the degree to which the non-uniformity of the heat flux influences the local HTC’s, and to develop correlations that may aid in predicting these cases. An integration with flow pattern mapping may also be done to solidify the understanding of the phenomenon governing these observations. / Dissertation (MEng)--University of Pretoria, 2021. / Department for International Development (DFID) through Royal Society-DFID Africa Capacity Building Initiative. / The UK Engineering and Physical Sciences Research Council (EPSRC) [grant numbers EP/T03338X/I and EP/P004709/1]. / Russian Government "Megagrant" project 075-15-2019-1888. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Modern nuclear reactors still use Zirconium-4 Alloy (Zircaloy®) as the cladding material for fuel elements. A substantial amount of research has been done to investigate the boiling heat transfer behind the cooling mechanism of the reactor. Boiling heat transfer is notoriously difficult to quantify in an acceptable manner and many empirical correlations have been derived in order to achieve some semblance of a mathematical model. It is well known that the surface conditions on the heat transfer surface plays a role in the formulation of the heat transfer coefficient but on the other hand it also has an effect on the pressure drop alongside the surface. It is therefore necessary to see whether there might be an optimum surface roughness that maximises heat transfer and still provides acceptably low pressure drop. The purpose of this study was to experimentally measure pressure drop and heat transfer associated with vertical heated tubes surrounded by flowing water in order to produce flow boiling heat transfer. The boiling heat transfer data was used to ascertain what surface roughness range would be best for everyday functioning of nuclear reactors. An experimental set-up was designed and built, which included a removable panel that could be used to secure a variety of rods with different surface roughnesses. The pressure drop, surface temperature, flow rate and heat input measurements were taken and captured in order to analyse the heat transfer and friction factors. Four rods were manufactured with different roughnesses along with a fifth rod, which remained standard. These rods were tested in the flow loop with water in the upward flow direction. Three different system mass flow rates were used: 0kg/s, 3.2kg/s and 6.4kg/s. Six repetitions were done on each rod for the tests; the first repetition was not used in the results since it served the purpose to deaerate the water in the flow loop. The full range of the power input was used for each repetition in the tests. For the heat transfer coefficient at a system mass flow rate of 3.2kg/s, satisfactory comparisons were made between the test results and those found in literature with an average deviation of 14.53%. At 6.4kg/s system mass flow rate the comparisons deviated on average 55.45%. The velocity of the fluid in the test section was calculated from the pressure drop and was validated using separate tests. The plain rod, with no added roughness, was found to be the optimal surface roughness which is what is used in industry today. The flow loop was in need of a couple of redesigns in order to produce more accurate results. Future work suggestions include adding more rods in the test section in order to investigate the nature of heat transfer in a rod bundle array as well as implementing all the suggested changes listed in the conclusion. / Dissertation (MEng)--University of Pretoria, 2017. / Mechanical and Aeronautical Engineering / MEng / Unrestricted