Doctoral Candidate Hristina Dragovic at the Department of Energy and Process Engineering will hold a trial lecture and defend her doctoral thesis for the degree of Philosophiae Doctor (PhD). The degree is administered by the Faculty of Engineering at the Norwegian University of Science and Technology (NTNU).
Thesis title
Experimental investigation of the transport of water vapor and liquid in porous insulation materials
for an electronic version of the thesis, please contact nina.laumann@ntnu.no
Trial lecture
Energy Systems for a Net-Zero Norway: Heavy Industries
Assessment committee
The Faculty has appointed the following Assessment Committee to assess the thesis:
1st opponent: Professor Dirk Lars Engelberg, University of Manchester, UK
2nd opponent: Specialist Zhilin Yang, Equinor, Norway
Administrator: Associate Professor Chirag Trivedi, NTNU
The committee has concluded that the thesis is worthy of public defense for the PhD degree.
Main supervisor: Professor Ole Jørgen Nydal, Department of Energy and Process Engineering
Co-supervisors
Researcher Åsmund Ervik, SINTEF Energy
Dr. Daniela da Silva Damaceno, SINTEF Energy
Time and venue
Trial lecture: 31 August 2026 at 10:15 in Disputasrommet, Hovedbygget, NTNU Gløshaugen
Public defence: 31 August 2026 at 13:15 in Disputasrommet, Hovedbygget, NTNU Gløshaugen
You can follow the trial lecture and defence via Zoom by clicking on this link
Abstract of thesis
The aim of this thesis is to identify the heat and mass transfer mechanisms governing the drying behavior of insulation material and to advance the understanding of their interactions during prolonged drying, with the objective of improving drying efficiency in industrial applications and thereby reducing the risk of corrosion under insulation (CUI). CUI arises when water is retained within insulation systems due to ingress from the ambient environment or condensation of humid air trapped within the system. Persistent moisture promotes corrosion of underlying metallic components, decreasing structural integrity and operational reliability. CUI is an external corrosion phenomenon, and many studies have attempted to apply principles from general external corrosion research to CUI. However, these approaches have often been less successful. Relying solely on conventional external corrosion principles to understand and mitigate CUI, without accounting for its hidden and less obvious behaviors, does not lead to meaningful improvements in risk-based assessment strategies. Reducing CUI risk enhances long-term performance and sustained operation of energy-producing systems, including power plants, oil refineries, chemical processing facilities, and offshore oil and gas installations. Moreover, improved insulated equipment’s drying performance reduces maintenance frequency and associated material disposal, thereby mitigating environmental impacts.
Repeated wetting and drying cycles are known to significantly accelerate CUI progression. Therefore, this thesis also examines the coupled heat and mass transfer processes associated with phase change within insulation materials under temperature gradients and relative humidity conditions representative of industrial environments. Thus, experimental work specifically focuses on studying condensation and evaporation, since additional liquid water requires more time for re-evaporation. The work focuses on mineral wool insulation, a widely used material in industrial facilities. The topic of heat and mass transfer including phase change within insulation has been extensively investigated through both modeling and experimental studies in the field of building physics. In this context, identifying phase change processes within insulation materials is essential for improving building energy efficiency, preventing mould formation, and maintaining desired indoor environmental conditions. However, literature reviews reveal significant gaps in data on humid air within mineral wool insulation exposed to the strong temperature gradients typical of industrial applications, and phase change processes in these system have not been adequately studied. This study is the first to systematically investigate the transport of water vapor from the location of evaporation, through mineral wool insulation, toward the cladding during alternating wetting and drying cycles. It further demonstrates how liquid water forms on the cold cladding surface due to condensation, and the extent to which this condensation can spread, potentially allowing liquid water to reach the hot metal surface and re-evaporate. The present work therefore represents a significant step forward in understanding whether water completely escapes the system after each wet–dry cycle or partially remains trapped within the insulation fibers.
Particular attention is given to the influence of geometry, as certain configurations of insulated assets are more prone to moisture retention and delayed drying. We have through this work tried to answer how different geometrical arrangements, amounts of water evaporating into the mineral wool insulation, pre-exposure of the material to the humid air, affect moisture accumulation and drying rates, with the aim of identifying designs and applications that promote more effective moisture removal. We have also explored how identifying the governing mechanisms of moisture transport and phase change contribute to improved predictive capability of insulation drying performance using sensor monitoring technology.
To address these issues, we designed a methodology and constructed experimental rigs for measuring the temperature and relative humidity of the humid air within mineral wool insulation using sensors, specifically, the specific humidity, during evaporation and drying on the hot vertical pipe. By inducing evaporation of varying amounts of liquid water added from the reservoir, the drying times of the insulation material were determined. Moreover, to closely examine whether water completely leaves the system, the rebuilt vertical rig was equipped with a weighing scale to quantify any liquid water accumulation following the evaporation of varying amounts of added water. In addition, an NMR technique was employed to measure moisture content in the mineral wool sample during condensation, under applied temperature gradient and varying relative humidity conditions of the humid air inflow. Additional experiments were conducted using dynamic vapor sorption (DVS) to evaluate the potential of mineral wool for water vapor adsorption and the formation of liquid water films.
We have in this work established a first set of experimental data for drying time of the mineral wool insulation material depending of different amounts of water evaporating into it. We have found that amount of liquid water added, time between subsequent water addition events, insulation system geometry (drainage opening size or fully closed system, insulation thickness) mostly affect the length of the insulation material drying. In terms of condensation, we have shown that condensation area width and moisture content increase due to relative humidity increase at the inlet to the insulation material, and that supersaturation is needed for nucleation of droplets initialization. The DVS measurements revealed that mineral wool exhibits low water sorption, reaching a maximum of 0.3\% at 98\% relative humidity. In particular, with respect to repeated wetting and drying cycles, experimental results demonstrate that pre-exposure of mineral wool insulation leads to progressively longer drying times. Experiments conducted on the reconstructed vertical rig are considerably longer, providing a time scale for all relevant mechanisms, especially adsorption of gases to manifest, as reaching equilibrium is an inherently slow process. In that sense, repeated wetting tests have a notable consequence: the secondary drying time cannot be exactly the same, even when eight months separate subsequent evaporations of the same amount of liquid water. This indicates that in applications where wetting and drying occur repeatedly, each subsequent wetting event results in a prolonged drying period. Such cumulative effects are unfavorable from a corrosion perspective, as extended moisture residence time within the insulation increases the likelihood and severity of CUI.
The results in this thesis show that there are pathways to improving monitoring and maintenance strategies by identifying the areas in the system that are under prolonged wetting conditions, and thus,with high risk of CUI. The findings show that: 1) thinner insulation layers, 2) increased size of the drainage slit, 3) lower amounts of liquid water evaporating into the insulation, 4) single wetting (avoiding pre-exposure) of the insulation material, are resulting in lower insulation material drying times. The adsorption of liquid water films and the presence of condensed liquid water were identified as the primary mechanisms responsible for the prolonged drying time of insulation materials, representing an additional source of liquid water that requires time to re-evaporate. We have also demonstrated monitoring relative humidity, particularly specific humidity, can reveal the point within the mineral wool where condensation of supersaturated water vapor begins, between the cold cladding and the pipe wall. Recognizing the dry zone during supersaturated water vapor flow, where adsorbed water films start to form, offers essential
information for estimation of the drying time in sensor-based monitoring. The findings from this work represent a significant advancement in maintenance data collection, thereby improving strategies for CUI prevention. The results demonstrate that thermodynamic properties of humid air can inform CUI risk assessments and validate a sensor-based technique for measuring specific humidity in mineral wool insulation to estimate drying time, an essential indicator for CUI inspection, while offering a novel approach for detecting wet zones and guiding targeted maintenance.