Arid desert regions of western India receive a high level of direct solar irradiance, which is desired for the concentrated solar thermal technologies. For instance, the heliostats are used to concentrate beam solar radiation onto an open volumetric air receiver placed on the top of a solar tower. This receiver comprises of porous absorbers that are exposed to a high heat flux (say > 800 Suns) and are open to atmosphere. Such a receiver is developed and evaluated at IIT Jodhpur. For the unsteady heat transfer in this receiver the seven equations based one-dimensional zonal model is deduced. This is the novelty in this comprehensive two-zone model including the heat transfer in receiver casing/enclosure. In this model the absorbers are categorized into central and peripheral types. In this model the instantaneous energy equations are solved in solid and fluid domains. For the numerical solution a fully implicit scheme, which is second order in space and first order in time is implemented. This paper presents (a) the developed zonal model for unsteady heat transfer in the receiver, and (b) its validation using in-house experimental data. The model will allow improvement and scale-up of the receiver design for industrial applications.