## 1 Introduction  
Over the past decade, the use of phase change material (PCM) as thermal energy storage (TES) medium has received significant research attention. Due to the larger equivalent heat capacity compared to single phase medium and better heat transfer characteristics compared to gas phase, slurries are best suited as a heat transfer fluid and TES medium [1; 2; 3; 4; 5]. Among numerous phase change materials, ice slurries are one of the commonly available phase change material. Ice slurries are recognized as suitable materials for TES and they have already been used in many practical processes [7; 8]. Interest in gas hydrates technology has also grown drastically, because of its diverse disciplines, including the potential utilization as effective phase change material for TES medium [9; 10; 11]. Clathrate hydrates are ice-like crystalline inclusion compounds that are formed even at above zero temperature through the hydrogen bonding of water molecules, with the assistance of gas molecules if the pressure is high enough [12; 13]. Hydrate slurry is a relatively new type of phase change medium, in which the solid phase is clathrate hydrate particles, and the liquid phase is liquid water, which serves as the carrying fluid [14]. The transportation of aqueous hydrate slurry over a range of piping network is vital for almost all industrial applications of hydrate technology [15; 16]. The solid volume fraction of hydrate slurry needs to be known to estimate the thermo-physical properties, and to obtain flow assurance. The solid fraction of the slurry can be measured using Nuclear Magnetic Resonance spectroscopy (NMR) based on the different magnetization decay rates of solid and liquid. In the food industry, estimation of slurry solid content is essential, because it not only dominates rheological factors, but also influences the texture of product, and consequently consumer acceptance [17; 18]. Time domain (TD) NMR is considered to be a reliable experimental diagnostics to measure the slurry fat contents related to the food industry. TD-NMR has been employed to quantify the crystalline phase fraction in polysaccharides [19; 20; 21], lipids [22; 23; 24], and ice cream [25; 26]. The solid fat contents of oils were also measured using low-resolution pulsed NMR [27]. Studies on the evaluation of slurry solid fraction from measured electrical properties of two phase fluid have been reported [28; 29]. Electromagnetic (EM) attenuation is another phenomenon that can be exploited to measure the solid fraction, since the microwave absorption of the solid is different from that of the liquid. Utilizing neutrons [30; 31], gamma ray [32; 33; 34], X-ray [35] and microwave [36] solid volume fractions could be determined without invasion. [PERSON] et al. [37] applied the EM attenuation technique to measure the solid fraction of ice and brine slurry during the ice pigging process, where the shear stresses on the pipe walls need to be monitored closely for safe operation. The solid fraction is a dominant parameter influencing non-Newtonion flow behavior of the clathrate hydrate slurry [38; 39; 40; 41; 42], which imposes challenges in designing flow systems, as well as in estimating their performance. Extensive research efforts have focused on developing novel techniques to estimate the solid fraction of the hydrate slurries. The traditional method to theoretically determine the solid fraction of hydrate slurries is momentum law based phase diagrams [43; 44]. [PERSON] et al. [45] proposed an experimental method to determine the hydrate fraction based on ultrasonic interpretation. Due to its destructive nature, ultrasonic technique is not compatible with the industrial on-site setup. Other methods are based on optical properties, such as image analysis, or near infrared spectrometry [46]. The limitation associated with such techniques is the requirement for frequent calibration.